When crossing a front,isobars tend to（）. A
题型：单项选择题
问题：
When crossing a front,isobars tend to（）.
A.change from smooth curves within the air mass to sharp bends at the front
B.change from sharp bends within the air mass to smooth curves at the front
C.pass smoothly across the front with no change
D.become closer together at the front and pass through in straight lines