下列操作过程中必须使用托盘天平的是（　　）

A．用加热法测定硫酸铜晶体里的结晶水数目

B．将体积分数为99%的乙醇稀释成体积分数为75%的医用酒精

C．用98%的浓硫酸配制500克质量分数为10%的稀硫酸

D．滴入适量的氯化钡溶液将3毫升某溶液里的硫酸根离子沉淀完全

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