长冠基牙正确概念是（）。

A.在牙龈缘上保留0～8mm牙冠的基牙

B.在牙龈缘上保留1～8mm牙冠的基牙

C.在牙龈缘上保留2～8mm牙冠的基牙

D.在牙龈缘上保留3～8mm牙冠的基牙

E.在牙龈缘上保留8mm牙冠的基牙

Questions 19 to 22 are based on the conversation you have just heard.

A．It’s crowded with people.

B．It’s full of computers.

C．Lunch is being served there.

D．Only few people in there.