知道话务与GOS计算信道，笔算怎么算？

Aparticular cell is expected to support 6 erlangs of traffic at 1% Grade of service? How

many traffic channels are required?

A、6

B、13

C、3

D、15

E、22

用工具算出来大概需求12个TCH，但与答案不对，我想知道笔算怎么算，麻烦下！