问题
选择题
_____ 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
答案：C
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。