need doing 和 need to be done 中,need 是实义动词,need 作为实义动词时不可以与 not 直接连用,再接动词时要接非谓语动词,即：作为实义动词 needn't 不存在,所以原句中B答案 needn't to be answered 要么是 needn'tbe answered,要么就是doesn'tneed to be answered.
另外,如果句子后面用 by 短语附带“由某人去完成”时,通常用 need to be done.例如 ：
The door needs to be painted by a skilled worker. (这句不可用 need doing)