关于食管描述正确的是（）

A.分为颈部和胸部

B.第一个狭窄平对C5水平

C.第二个狭窄在左主支气管交叉处

D.第二个狭窄位于食管穿过膈的食管裂孔处

E.第三个狭窄是食管癌好发部位

why do some stations keep silence?（）

A.Because they are not engaged in the distress traffic

B.they will affect the transmission of the distress traffic

C.The powers of these stations are very weak

D.They are not in charge of search and rescue