进行二尖瓣替换手术时，容易损伤哪支冠状动脉（）

A.右冠状动脉

B.对角支

C.前降支

D.回旋支

E.左冠状动脉主干

The loran lines drawn on navigation charts represent（）.

A.Ground waves

B.Skywaves

C.Either ground waves or skywaves interchangeably

D.An average between ground wave and skywave positions