If anything _________ the machine,please let me know.A.happens to B.happens on C.happened to D.happened on

选A
用happens to,全句意思“如果机器出了什么问题,请通知我.” happen to sb/sth意思是“出了（问题）,发生（意外）”,通常指“出了意想不到的（坏）事情”.又如：An accident happened to him.他出了事故.happen on意思是“偶然碰到,偶然发现”,例如：The two men happened on each other by chance.那两人偶然碰了面.