Question

which of the following would be the best frame of reference when measuring the speed of a car?
the seat
the driver
a sign post
an airplane

Explanation:

A frame of reference should be stationary relative to the ground for measuring a car's speed. The seat and driver move with the car, and an airplane is not a practical or stationary - relative - to - ground reference. A sign post is fixed on the ground, providing a stable reference for measuring the car's motion.

Answer:

C. A sign post