问题补充：
20. that caused him to serve dinner an hour later than usual.
A. It was we being late
B. It was our being late
C. It was we were too late
D. It was because we were late
答案：
A. It was we being late
(答案→)B. It was our being late
C. It was we were too late
D. It was because we were late
解析：此题容易认为强调的是原因状语从句 because we were late。但实际上，此题的答案为B，强调的是句子主语 our being late，此题若还原成非强调句，即为：Our being late caused him to serve dinner an hour later than usual. 注意，强调句的一个显著特点是，若去掉强调结构 it is [was]… that…，句子仍然成立；换句话说，该结构中的 it is [was]… that… that 不能充当句子成分，而实际上句中的动词cause需要有自己的主语。
