I know one of the things I was unsure about was hard it would be to climb this mountain.A

问题补充：

I know one of the things I was unsure about was hard it would be to climb this mountain.

A. what

B. that

C. however

D. how

答案：

A. what

B. that

C. however

(答案→)D. how

解析：考查感叹句：hard是形容词，用how修饰，what修饰名词，这句话是表语从句，如果不是感叹句应该是It would be hard to climb this mountain。对hard强调用how，句意：我知道我不能确定的一件事情是爬这座山会多难，选D。