某DNA片段转录的m RNA中尿嘧啶占28%，腺嘌呤占18%，则这个DNA片段中胸腺嘧啶和鸟嘌呤分别占(    )

A.46%  54%                                 

B.23%  27%

C.27%  23%                           

D.46%  27%