Nacre is a substance that is ______

A．mechanically manufactured

B．the result of laboratory testing

C．organically secreted by the mollusk

D．present in the chemical composition of freshwater pounds

粘性流体总水头线沿程的变化是：（）

A.沿程下降

B.沿程上升

C.保持水平

D.前三种情况都有可能