I've had some difficulties, but they were nothing ________ yours.[ ]

A．compared to  

B．compares with

C．comparing to  

D．comparing with

箱子里有大小相同的3种颜色玻璃珠各若干颗，每次从中摸出3颗为一组，问至少要摸出多少组，才能保证至少有2组玻璃珠的颜色组合是一样的？

A.11 

B.15 

C.18 

D.21