We haven't known ____ there is life on Mars.A.that B.if C.what D.and

B我们还不知道火星上有没有生命.there is life on Mars是一个宾语从句,而and是引导并列分句的,故排除D.there is life on Mars是一个意义十分完整的宾语从句,不缺任何成分,而what引导宾语从句时必须在从句中充当主语...