问题补充：
Mary waited here (for three hours).对括号内提问 ————— ————— ————— Mary
答案：
How long did Mary wait here
======以下答案可供参考======
供参考答案1:
How many hours did wait
有不会的可以再问我
供参考答案2:
——How——— ——long——— ——did——— Mary _wait____ here?
供参考答案3:
Mary waited here (for three hours).对括号内提问 How long did Mary wait here?
