全冠修复体采用龈上边缘的最主要优点是（）
A.不易附着菌斑
B.美观性好
C.边缘密合
D.对龈刺激小
E.不易附着牙垢
参考答案：D
某有机物的结构简式为：CH2=CHCOOH，它含有的官能团正确的是[ ]
① -CHO ②-OH ③ ④-COOH
A．③④
B．②③④
C．①②④
D．①②③④
–Did you _____ your sister when you were in Shanghai?
--No. But I _____ my uncle’s during my stay there.
A．call on, called on
B．call at, called at
C．call on, called at
D．call at, called on
④-COOH