固定义齿修复最适宜的年龄段是（）。
A.12岁以上
B.15～19岁
C.20～60岁
D.60岁以上
参考答案：C
Many a new measure ________put forward to reduce air pollution so far. [ ]
A. have been
B. has been
C. was
D. were
决定酶催化活性的因素有两个方面，一是（），二是（）。
