A bike is moving north when the rider applies the brakes. In which direction does the frictional force act between the ground and the bike's wheels?

A.
north

B.
south

C.
upward

D.
downward

As light moves from one material into the next, which of the following affects how much the light waves will refract, or bend? angle at which the ray strikes the medium color of the material density of the material temperature of the light wave