保卫叶表皮细胞的一般特征是

A．细胞成正方形，排列紧密

B．细胞呈半月形，内含叶绿体

C．细胞圆柱状，内有叶绿体

D．细胞无色透明，外壁有角质层

What two factors help protect the Earth from radiation

A．Magnetic field and atmosphere.

B．Rapid spin and molten iron-nickel core.

C．The Sun and the Moon.

D．Blue waters and white clouds.