【题目】We still have the question of ________ they have discovered the trade secret.
A.what
B.why
C.how
D.that

【答案】C
【解析】考查宾语从句。句意：我们仍然对他们如何发现这商业秘密有疑问。从句中主谓宾齐全，所以空格处只能作状语或起连接作用，故排除A项；C项符合语境。