Chinook Winds Frequently Bring Warm Temperatures to the Great Plains,even

Chinook winds frequently bring warm temperatures to the Great Plains,even in the middle of winter.Why are these winds so warm?

A) Volcanic activity in the Rockies warms the air before it reaches the plains.
B) Air as it descends tends to expand causing a warming effect.
C) Warm air from the Gulf of Mexico meets the cold,dry air from the Arctic.
D) Increasing pressure warms the air as it descends down the slopes of the Rocky Mountains.