Many scientists say that it’s important for us to have ______ food in our diet.

A．a variety of

B．a great deal

C．a large number of

D．a good many

成熟菌斑的基本结构分为（）。

A.获得性膜和唾液

B.基底层、中间层和表层

C.上皮细胞和羟基磷灰石

D.多糖和矿物质

E.唾液蛋白、细菌和矿物质