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。
