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There is an old saying in English: "Laughter is the best medicine". Until recently, few people took the saying very seriously. Now, however, doctors have begun to investigate laughter and the effects it has on the human body. They have found evidence that laughter really can improve people’s health.Tests were carried out to study the effects of laughter on the body. People watched funny films, while doctors checked their heart rate, blood pressure, breathing and muscles. It was found that laughter has similar effects to physical exercise. It increases blood pressure, the heart rate and the rate of breathing; it also works several groups of muscles in the face, the stomach, and even the feet. If laughter exercises the body, it must be beneficial.Other tests have shown that laughter appears to be capable of reducing the effect of pain on the body.There is also some evidence to suggest that laughter helps the body’s immune system, that is, the system which fights infection.As a result of these discoveries, some doctors and psychiatrists (精神病学家)in the United States now hold laughter clinics, in which they try to improve their patients’ condition by encouraging them to laugh.1) Laughter really can (46) .2) In the first test, it was found that laughter has similar effects to (47) .3) Other tests have shown that laughter can reduce the effect of (48) on the body.4) There is evidence to show that laughter helps the body’s (49) .5) In the clinics, the psychiatrists try to improve their patients’ condition by encouraging them (50) . 46（）

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阅读以下说明和关系表，回答问题1-6。[说明] 设有三个关系学生S(学号，姓名，性别，年龄，籍贯) 课程C(课程号，课程名，任课老师，所用教材) 学生成绩SC(学号，课程号，得分) 其中规定各属性定义如下：学号：SNUM 姓名：SNAME 性别：SEX 年龄：AGE 籍贯：NATIVE 课程号：CNUM 课程名：CNAME 任课老师：TEACHER 所用教材：BOOK 得分：GRADE 试用关系代数表达式表示下列查询语句。 [问题3] 检索全部学生都选修的课程号。

火山爆发与减少耕地面积之间有联系吗 1982年2月底至3月，墨西哥爱尔·基琼火山爆发。据报道，这次火山爆发飞向天空的火山灰数量是观测史上数量最大的一次，这对自然界产生了重大的影响。一方面，大量的火山灰喷入空中，遮蔽了自然日光，山太阳发出的光能就会被反射回去，从而使得那些需要阳光的地球生物无法获得充足的太阳能。另一方面，由于火山灰云笼罩了整个地球，仿佛给地球盖了个天棚，这个天棚会使本来应从地球向外散发的某些能量反射回地球。结果世界各地几个典型地区产生了显著的气象变化。 1983年冬，某北极考察船在北极被困住。往常根本不会结冰的地方，这回竟结冰封冻了，甚至连前去救援的破冰船也进退不能。大量火山灰飞向天空，使空气有了大量水蒸气变成水滴的核，平时不会变成水滴的水蒸气也变成了水滴，结果造成沿海地区大量降雨，像英国、法国海岸、西班牙、葡萄牙等这样一些本来就是多雨的地区，降雨量进一步增加。伦敦降雨27天，为历史所少见。与沿海地区的多雨相反，在那些内陆地区却出现了严重的干旱。季风刮来，在沿海地带雨水就已降下，季风无法再将水分带向内陆地区，于是干燥的空气被送往内地。结果伴随着西欧沿海国家的多雨，东欧三国出现了有史以来的第一次大旱。大雨、干旱、寒冷必然会导致全世界范围的农业歉收。对气象问题颇有研究的美国拥有气象卫星，依靠这些卫星，可以对全世界的农业进行监视。所以，对于上述种种气象变化所导致的世界各地农业不振的局面，当然也是清楚的。但就在此时，美国却推出了一个减少1/3耕地面积的决定。美国是在明知世界各地的农业将出现不景气的情况下，决定减少耕地面积的。据说在施行这一决定时，美国官方还采取一旦农民需要，立刻还给他们减少的那部分粮食的做法。毫无疑问，结果是谷物价格上涨，芝加哥的谷物市价比以往上升了1.6倍。此外，由于谷物价格的提高，农民的收入随之增多。这给与农业相关的行业，如农具、化肥、农药等也带来了新的转机。事实上，早在1982年，美国就将粮食作为战略物资，试图禁止对苏联的出口，但结果却失败了。在爱尔·基琼火山爆发引发世界气候大变化之前，除美国外还有一些国家也具有粮食出口能力。例如，苏联从阿根廷进口粮食，就曾使美国出现大量的粮食剩余，其价格直线下降，引起农民和一部分人的不满。世界农业大面积歉收，最具粮食出口能力的美国减少1/3的耕地面积，这使得依赖粮食进口的苏联必须花更多的外汇换取粮食。这样，1982年美国遭受的损失，又借助天气赚了回来。所以有人把美国的这一决定视为利用气象因素实现一箭多雕的绝好范例。然而，美国的决定是否真的明智和成功呢根据上述情况，请回答下列问题：美国政府的上述决策主要考虑哪两个方面的环境因素( )

A. 国内外自然与政治环境
B. 国内经济和自然环境
C. 国外政治环境与国内经济因素
D. 国外经济与政治环境

阅读以下说明，回答问题1-4。[说明] 设学校教学数据库的模式如下： S(SNUM，SNAME，AGE) SC(SNUM，CNUM，CREDIT，SCORE) C(CNUM，CNAME，TEACHER) T(TNUM，TEHCHER，POS，SAL) SNUM表示学生的学号，SNAME表示学生的姓名，AGE表示学生的年龄，CNUM表示选修课程的编号，CREDIT表示该课程的学分，SCORE表示课程的成绩，CNAME表示课程名，TEACHER表示任课的教师,TNUM表示老师的工作证号，POS表示教师的职称，SM，表示教师的收入。试定义下列完整性约束和规则： [问题2] 在关系SC中插入的元组时，其SNUM和CNUM的值必须分别在S和C中出现过。

There is an old saying in English: "Laughter is the best medicine". Until recently, few people took the saying very seriously. Now, however, doctors have begun to investigate laughter and the effects it has on the human body. They have found evidence that laughter really can improve people’s health.Tests were carried out to study the effects of laughter on the body. People watched funny films, while doctors checked their heart rate, blood pressure, breathing and muscles. It was found that laughter has similar effects to physical exercise. It increases blood pressure, the heart rate and the rate of breathing; it also works several groups of muscles in the face, the stomach, and even the feet. If laughter exercises the body, it must be beneficial.Other tests have shown that laughter appears to be capable of reducing the effect of pain on the body.There is also some evidence to suggest that laughter helps the body’s immune system, that is, the system which fights infection.As a result of these discoveries, some doctors and psychiatrists (精神病学家)in the United States now hold laughter clinics, in which they try to improve their patients’ condition by encouraging them to laugh.1) Laughter really can (46) .2) In the first test, it was found that laughter has similar effects to (47) .3) Other tests have shown that laughter can reduce the effect of (48) on the body.4) There is evidence to show that laughter helps the body’s (49) .5) In the clinics, the psychiatrists try to improve their patients’ condition by encouraging them (50) . 50（）