单项选择题
A.loxodromic curve
B.curved line concave to the equator
C.straight line
D.spiral approaching the poles as a limit
单项选择题 Mariners proceeding across the main routes are recommended to do so at（）.
单项选择题 Mariners proceeding across the main routes are（）to do so at as wide an angle as practicable.
单项选择题 Mariners are reminded that they should proceed in the general direction indicated by the arrows or，if（）a lane，they should do so as nearly as practicable at right angles to it.