The doppler effect means that an observer ahead of a train hears the train at a higher frequency sound than does any observer behind the train.
true or flase

True.

Explanation:

The Doppler Effect, named after the Austrian physicist Christian Doppler, is the change of frequency concerning an observer and depending on where is he/she.

The reason that the statement is true is that the observer who is ahead of the train receives the waves closer and closer while the train is moving, while the observer behind the train receives the waves at a more reduced time as the train gets away.

I hope this answer helps you.

as resources become scarce because of population growth, the growth decreases after few years due to lack of resources and food.

"a.) well–developed vascular system."