A company that ships boxes to a total of 12 distribution centers uses color coding to identify each center. If either a single color or a pair of two different colors is chosen to represent each center and if each center is uniquely represented by that choice of one or two colors, what is the minimum number of colors needed for the coding ？（） (Assume that the order of the colors in a pair does not matter.)

A．4

B．5

C．6

D．12

E．24

子宫肌瘤的症状主要取决于（）。

A.肌瘤大小

B.肌瘤数目

C.肌瘤质地

D.肌瘤与肌层关系

E.病人年龄