问题补充：
---Why did the teacher get angry? --- what we had done the day before.
A. Because
B. Since
C. As
D. Because of
答案：
A. Because
B. Since
C. As
(答案→)D. Because of
解析：考查介词短语。because of 是介词短语，后接名词、代词或者接名词性从句。而在此处是有what引导的名词性从句，相当于一个名词，所以用because of。
