18.. In the Northern hemisphere, if a pilot makes a long-distance flight from east to west, under which of the following conditions, encounter against the wind forming by air pressure system?
选项：

A:
A: when the airplane is in the south of high pressure system and in the north of low pressure system.
B:
B: When the airplane is in the north of high pressure system and in the south of low pressure system.
C:
C: When the airplane is in the north of high pressure and low pressure systems .