The instrument ____ was not in the laboratory.
问题描述：
The instrument ____ was not in the laboratory.
A.which the professor referred us
B.to which the professor referred us
C.to which the professor referred to us
D.which the professor referred to us
答
选D.固定用法refer sb/sth to sb/sth.此处refer the instrument to us.定语从句中用which 代替 instrument .