Did you read a book about customs last week?-- Yes I found very useful to know the c

问题补充：

-- Did you read a book about customs last week?

-- Yes, I found very useful to know the customs of foreign countries.

A. that

B. this

C. it’s

D. it

答案：

A. that

B. this

C. it’s

(答案→)D. it

解析：句意：上周你读了一本关于风俗的书吗？是的，我发现了解外国的风俗习惯非常有用。Find + 名词 + 形容词，是find的一种习惯用法，其中名词作宾语，形容词作宾补。此句中to know the customs of foreign countries是真实宾语，为了使得句子结构平衡，就将其放在了后面，让it来代替它充当宾语。所以选D。