单项选择题

某地市购买IP话机,数量如下: 
VVX1500:10部 
CP7975:30部  
C.CP7911:50部  
D.CP7906:100部 
在购买话机的同时,需要购买的话机许可DLU数量为:()

A.510
B.550
C.560
D.660