When the students came into the classroom, the teacher ______ on the blackboard.
A. was writing
B. is writing
C. wrote
My mother asked me ______ her with the cooking.
A. helps
B. to help
C. helping
消化性溃疡的流行病学调查正确的是
A. 我国十二指肠壶腹部溃疡和胃溃疡发病率相同
B. 女性与男性之比约为3~4:1
C. 60岁以上老年人一般不发生
D. 十二指肠壶腹部溃疡近年来发病率下降
E. 胃溃疡多见于胃角和胃窦
二氧化钛
A. 成型材料
B. 增塑剂
C. 避光剂
D. 防腐剂
E. 增稠剂