参考答案

正确答案：B
解析：动名词、不定式和从句做主语时，谓语动词一般用单数。例如：To see is to believe. (百闻不如一见。)Swimming is a good way to keep health. (游泳是个健身的好办法。)Who is her father is not known. (谁是她的父亲尚不得知。)在本句中主语从句做主语，因此动词要用单数形式。