He was elected --- A a monitor B to be a monitor C the monit

elect 作为动词有选举,推选；选择,决定的含义.后可直接加职务.
如：elect sb sth ,elect sb to be sth,elect sb to be the sth.
题中B答案to be 有将来的含义,而题干中是过去时表明他已被选为班长.不需要将来含义.
而C答案加上了定冠词the表示特指.题中也不需特指某个班长.
所以答案应该选D