【Hewasoftenheard()inhisapartmentbefore,butnow,noonecanhearhim().A.tosing;singB.sing;sangC.singing;singingD.sing;singing并说明一下原因】

　　Hewasoftenheard()inhisapartmentbefore,butnow,noonecanhearhim().

　　A.tosing;sing

　　B.sing;sang

　　C.singing;singing

　　D.sing;singing

　　并说明一下原因