The policy of the mainland is very clear and straightforward，and the key problem is the leaders of Taiwan will accept it.
A. that
B. how
C. why
D. whether
网友回答
A. that
B. how
C. why
(答案→)D. whether
解析：句意：大陆的政策很清楚也很直接，关键问题是台湾领导人是否接受。由句意可知，要用whether(是否)引导这个表语从句。