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
网友回答
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。