问题补充：
— What do you think of math?
— I find difficult to learn it.
A. that
B. this
C. its
D. it
答案：
A. that
B. this
C. its
(答案→)D. it
解析：句意：----你认为数学怎么样?---我发现学习它很难。考查固定句型结构“find+it+形容词+动词不定式”的用法。这个句型结构为：主语+谓语+it+形容词+动词不定式。形容词充当宾语补足语，补充说明it的情况或状态。it是形式宾语，动词不定式是真正的宾语。这样就避免了头重脚轻的现象，使句子平稳。故选D.
