引发牙髓感染的途径包括
A．暴露的牙本质小管
B．牙髓暴露
C．牙周途径
D．血源性感染
E．以上都是
参考答案：E
As he was about to go out to search for Mary, she happened ____.
A．to coming in
B．to come in
C．came in
D．to have come in
放线、紧线前，应检查确认线盘架（）。
A.稳固可靠
B.转动灵活
C.制动可靠
