关于happen,takeplace,comeabout用法区别以及total有这样一道题目Theyusedtobegoodfriendsbutnowtheyarelikestrangers.How()this(A.did;comeaboutB.did;happentoC.was;endedupD.was;happened应该选择A,可是不知道为什

　　关于happen,takeplace,comeabout用法区别以及total

　　有这样一道题目Theyusedtobegoodfriendsbutnowtheyarelikestrangers.How()this(A.did;comeaboutB.did;happentoC.was;endedupD.was;happened应该选择A,可是不知道为什么.所以想问一下这几个词语的用法区别在哪里.

　　(),theremusthavebeen20000peoplethere,thatis,thereare()20000peoplethere.A.Ontotal;atotalofB.Intotal;atotalofC.Intotal;thetotalofD.Ontotal;thetotalof为什么选择B,请麻烦解释一下这几个词的用法.ontotal/intotalatotalof/thetotalof