单选题
The number of connections C that can be made through a switchboard to which T-telephones are connected is given by the formula 
【正确答案】
B
【答案解析】连接有T门电话的交换机所能形成的连络数目C由公式[*]给定。30门电话能比20门电话多出多少连络?
30门电话的连络数目：[*]20门电话的连络数目：[*]
C30-C20=435-190=245
30门电话的连络数目：[*]20门电话的连络数目：[*]
C30-C20=435-190=245