青蛙皮肤裸露而湿润的意义是[ ]

A．保护体内水分不散失

B．有利于体表和外界进行气体交换

C．减少游泳时的阻力

D．适应水中生活，运动灵活

—Don't be angry with your kid when he makes a mistake again, will you?

—No, I won 't.  I know  that     of us  are perfect after all.

A．none

B．neither

C．each

D．all