The Pilot of an Aircraft Flies Due North Relative to the Ground

Multiple Choice

The pilot of an aircraft flies due north relative to the ground in a wind blowing 40 km/h toward the east. If his speed relative to the ground is 80 km/h, what is the speed of his airplane relative to the air?

A) 89 km/h
B) 85 km/h
C) 81 km/h
D) 76 km/h
E) 72 km/h