When I knocked at the door I heard her in the room .A. sings B. singing C. to sing

问题补充：

When I knocked at the door , I heard her in the room .

A. sings B. singing C. to sing

答案：

解析：本题考查非谓语动词。Hear sb doing sth听到某人正在做某事。句意：当我敲门的时候，我听到她在室内唱歌。