As he ________ in the river for five hours, the thief looked very pale and hopeless. [       ]

A. trapped  

B. has been trapped

C. trapping  

D. had been trapped

在用临界比例带法整定的临界比例带为PBK，振荡周期为TK，对PID调节器其PB和Td应分别为（）。

A.PB=PBS，Td=TK

B.PB=2.2PBS，Td=0.85TK

C.PB=1.7PBS，Td=0.125TK

D.PB=1.7PBS，Td=0.5TK