___________in the regulations that one should not tell other people the password of your email account.  [ ]

A. What is recommended            

B. What recommend    

C. It is recommended                

D. It recommend

如图所示，长L=75 cm的静止竖直筒中有一不计大小的小球，筒与球的总质量为4 kg，现对筒施加一大小为21 N、竖直向下的恒力，使筒竖直向下运动，经t=0.5 s时间，小球恰好位于筒口，求小球的质量。