Question

 (2011年南通一模)________there are living creatures in the outer space as those on Earth hasn’t been proved until now.

A．How                     B．When

C．Where                   D．Whether

Answer 1

解析：选D。考查名词性从句。句意：外太空是否存在像地球上一样的生物还不能确定。whether引导主语从句。