In vacuum, going at 2.99×10^8 m/s.
An educated guess to something you should test over and over
If a scientist is studying a model that is a widely accepted theory about the position of planets and sun, the model would most likely help the scientist to understand facts that can't be easily observed. That will make the correct answer to be C.
The mechanical energy of the roller coaster is sum of kinetic energy K and gravitational potential energy U:

where

is the kinetic energy

is the gravitational potential energy
Since the ride is frictionless, the total mechanical energy E is conserved during the ride. Therefore, at the top of the hill, the potential energy is maximum, because h (the height) is maximum, and this means the kinetic energy is minimum (because the sum of K and U is constant), so the velocity will be minimum. Viceversa, at the bottom of the hill, the potential energy will be minimum (because h is minimum), so the kinetic energy K will be maximum, and the velocity v of the roller coaster will be maximum.
V = IR
Where v is voltage I is current and r is resistance
So
V = 9
R = 12
V/R = I
9/12 = I
I = 0.75 A