It puzzles the scientists _____ some mammals produce their young _____ others lay eggs.

A．that; while

B．what; while

C．that; as

D．what; as

流域面积是指河流某断面以上（）

A、地面分水线和地下分水线包围的面积之和

B、地下分水线包围的水平投影面积

C、地面分水线所包围的面积

D、地面分水线所包围的水平投影面积