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锁相环是一个（）控制系统，它的基本原理是利用（）误差去消除（）误差，从而实现对输入信号频率的跟踪。
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关于数字签名，下面叙述正确的是（）
对现代网络信息系统的保护内容主要包括保护和（）
在不违反内部牵制制度的前提下，出纳员可以兼管（）
Do you often feel tired lately?Have a doctor said he can’t find anything wrong with you?Perhaps he advises you to take a physical examination,but all the advanced（高级的）equipment shows that there is nothing wrong with you.Then,consider this:You might be in a state of sub-health.Sub-health,also called the third state or grey state,is explained as a borderline state between health and disease.Experts have found out that people who work in management positions as well as students around the exam-week are more likely to suffer sub-health.They may feel depressed,anxious or under stress.Sub-health symptoms（症状）are a lack of energy,depression,slow reactions and poor memory.Some other symptoms may include shortness of breath,sweating and aching in the waist and legs.According to some medical experts,the key to preventing and recovering from sub-health is to form good living habits,alternate（结合）work and rest,exercise regularly,and take part in open-air activities.As for meals,people are advised to eat less salt and sugar.They should also eat more fresh vegetables,fruit and fish because they are rich in nutrient elements that are important to the body.Experts also point out that it is not good to eat too much at one meal because it may cause unhealthy changes in the digestive（消化）system.They also say that a balanced diet is very helpful in avoiding sub-health.The last but not the least,the help and understanding of family and friends are especially important.Sometimes life can become very much difficult to cope with.Then more help may be needed. When you are in a state of sub-health,your family and friends（）.
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