A:内鳃和外鳃
B:肺
C:气管、鳃和皮肤
D:鳃和肺
When the lights were red, we must stop ____ (walk).
如图是一块带有圆形空洞和方形空洞的小木板,则下列物体中既可以堵住圆形空洞,又可以堵住方形空洞的是( )
生物体能由小长大的原因是( )