SAT2物理题A woman standing on a train platform listens to a tra

SAT2物理题
A woman standing on a train platform listens to a train as it approaches her at a constant speed.The train engineer blows the whistle of the train which he knows to produce a frequency of 450 HZ.As the train approaches the woman on the platform,she will hear the frequency of the whistle as() A.greter than 450HZ but constant B.greater than 450HZ and steadily increasing 为什么选A呢?为什么是constant approach应该是 increasing frequency