My father was watching me --when someone knocked at the door

My father was watching me __when someone knocked at the door 1 分钟前 甜幂星月 | 浏览次数：7次
A plays the piano
B playing piano
C playing the piano
D play piano
答：C
译：我父亲正在看我弹钢琴,就在这时有人敲门.
析：本题考非谓语动词作宾补用冠词用法.
演奏乐器,一般用定冠词the,play the piano,排除BD
watch sb.doing sth.排除A.
评：出题者过于谨慎.如果楼主没有抄错的话,A项应该是play the piano才对.否则第二次排除就太简单了.因为plays the piano是谓语动词显然不对,不能说：watch me plays the piano的.
如果是play the piano,无to不定式,则要思考,为什么这里不能用watch sb.do sth.要从句意及逻辑上来判断.
watch sb.doing sth.是看某人在做某事.
watch sb.do sth.是看某人做了某事.