Einstein once refused to speak on the radio _____ $1,000 a minute. Another time he was seen using a

check _____ $ 1,500 as a bookmark. [ ]

A. for; of          

B. for; for          

C. at; for            

D. at; at

水内冷发电机在通水与不通水两种情况下，进行直流耐压试验各有何优缺点？