Charles is believed the floor yesterday; he is always helping others silently.
A. to sweep
B. to have swept
C. having swept
D. swept
网友回答
A. to sweep
(答案→)B. to have swept
C. having swept
D. swept
解析：考查非谓语动词。句意：相信查尔斯昨天已经打扫过地面了。他总是默默地帮助别人。Believe后用不带to的不定式，当变为被动语态时，要加上to表示具体的将要进行的动作，如果动作发生在谓语动词之前，则要用完成时态的不定式。结合语境可知打扫地板发生在相信之前，故选B.