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

对300MW及以上的发电机励磁绕组过负荷保护，正确的说法是( )。

A．定时限部分动作电流按照正常运行时最大励磁电流下能可靠返回的条件整定；
B．定时限部分只动作于信号；
C．反时限部分动作特性按发电机励磁绕组的过负荷能力确定；
D．反时限部分动作于解列灭磁，保护应能反映电流变化时励磁绕组的热积累过程。