问题补充：
The first thing we should do is to master the basic grammar.
A. which
B. that
C. as
D. of which
答案：
A. which
(答案→)B. that
C. as
D. of which
解析：定语从句考查题。先行词为The first thing因为有first修饰 thing所以关系词只能用that不能用which。
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