Mr. Brown has worked in the factory ______15 years.       [ ]

A. for                          

B. before                      

C. of                          

D. since

高级计量法在计量操作风险时认可保险的风险缓释影响，但保险的缓释作用不超过操作风险总资本要求的( )。

A．10%

B．15%

C．18%

D．20%