绵羊的白色由显性基因(R)控制，黑色由隐性基因(r)控制。现有一只白色公羊与一只白色母羊交配，生了一只黑色小羊。那只公羊和母羊的基因组成分别是 [ ]

A．Rr、Rr    

B．RR、RR    

C．RR、rr    

D．Rr、RR

This kind of computer is not very cheap, _____ it is veryexpensive.        [ ]

A. or else                          

B. on the contrary        

C. in short                           

D. on the other hand