When someone is knocking at the door,you may ask"______"?

--Listen! Someone is knocking at the door. There is someone knocking at the door.It may ________ (be) t Someone is knocking at the door.____ is it? Someone is knocking at the door,but who can____ be? Someone is knocking at the door._must be Tom someone is knocking at the door.who [ ]it be? --Listen! Someone is knocking at the door. Who _____it be? ---Listen! Someone is knocking at the door. Who _____it be? ---someone is knocking at the door.Who _____ it be? -----Someone is knocking at the door.It ____ be Tom . 英语翻译sorry，是Someone is knocking _____ ______ the door. There is someone knocking at the door.为什么用“knocking”,要加“ing”