Two trains are moving toward each other on two parallel tracks at the same speed with respect to the ground. When the whistle of one train blows, the frequency of the sound heard by a passenger on the other train will be
(a) greatest if the air is still.
(b) greatest if a wind blows in the same direction and at the same speed as the other train.
(c) greatest if a wind blows in the opposite direction and at the same speed as the other train.
(d) will not depend on whether there is a wind blowing.