The room very useful when we had visitors.
A. was proved
B. has been proved
C. has proved
D. proved
网友回答
A. was proved
B. has been proved
C. has proved
(答案→)D. proved
解析：prove=turn out 证明，结果是；可以作为系动词，后接形容词作表语，无被动形式。题目中用的一般过去时，故选D.句意：当我们有客人时，这个房子被证明是很有用的。