J2用来重复发送（）接入点识别符，以便通道接收端可据此确认它与预定的是否处于持续的连接状态。
参考答案：低阶通道
Galileo lived in the City of Pisa,()there is a leaning tower about 180 feet ().
A．in which; height
B．where; in height
C．which; high
D．where; height
