平行线间的单渡线，由两组相同号数的单开道岔和两道岔间的连接直线组成。
参考答案：对
伤寒患者排菌最多的时期是（）
A.起病后第1周
B.起病后第2～4周
C.起病前1周
D.起病后第5周
E.起病后第6周
2．--- Why were you absent yesterday?
--- I caught a cold, _______I had been walking around in the rain.
A．but
B．so
C．and
D．For
