We must take measures to protect the endangered wildlife ______ itis too late.  [ ]

A. before        

B. as long as      

C. now that      

D. unless

直放站增益调节步长要求（）Db。直放站增益调节步长误差是指实际增益步长与标称增益步长的差值，增益调节步长误差不超过（）/步长。