Is it the key to the door of for classroom

Is it the key to the door of for classroom Sorry,Miss Wang.I _____ the key to the door of the classroom the key to the door is missing is there a key to the door The key ____ for opening the door of the room. Sorry,I can't open the door,for I ( ) the key to it. the key to the door it′s very nice of you to help me find the key { }the door .A It is good for the students to get out of the classroom and the key is used by people for opening the door 】 英语种key的用法,使the key to the door还是the key of the door? key of the front door还是key to the front door?