Look! The light in the room is still on. Mr. Liu          at work.[ ]

A. must

B. can't be

C. must be

D. have to be

两牙之间都以近中面相接触的是（）

A.中切牙与中切牙

B.中切牙与侧切牙

C.侧切牙与尖牙

D.尖牙与前磨牙

E.前磨牙与前磨牙