请写出如图18所示的几个实验所揭示的物理道理：

甲：                                                                     

乙：                                                                     

丙：                                                                     

丁：                                                                     

检查肾小球滤过功能最恒定且灵敏的方法是（）

A.血肌酐测定

B.菊糖清除率

C.尿素清除率

D.血BUN测定

E.内生肌酐清除率