链球菌引起的尿路感染，有临床诊断意义的最低菌落计数为

A．清洁中段尿培养，菌落计数为500个/ml
B．清洁中段尿培养，菌落计数为1000个/ml
C．清洁中段尿培养，菌落计数为1万个/ml
D．清洁中段尿培养，菌落计数为5万个/ml