The doppler effect is a change in the observed frequency if there is relative motion between
a. two sources
b. two observers
c. a source and an observer

he is very short, and he can only reach halfway up on the elevator buttons. but on a rainy day, he has his umbrella with him, and he uses it to poke the button at the top.

yes

no

blue is scattered more than other colors because it travels as shorter, smaller waves. this is why we see a blue sky most of the time.