问题补充:
is known to all, the age children can go to school is seven.
A. What; which
B. It; when
C. As; at which
D. Which; when
答案:
A. What; which
B. It; when
(答案→)C. As; at which
D. Which; when
解析:考查引导词。As作为引导词一般用于句首意思为:如同。the age为下文逻辑时间状语,当前面有介词时,指物的引导词只能使用which,故选C。