I'm gonna beat you as soon as I finish what I            . [ ]

A. was doing        

B. am doing        

C. have done        

D. had been doing

由某DNA片段转录的mRNA中，尿嘧啶占26%，腺嘌呤占14%，则转录出此RNA的这个DNA中胸腺嘧啶和鸟嘌呤所占的比例（）

A.40%和60%

B.30%和20%

C.20%和30%

D.40%和30%