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Answer:
B) biker at 12 mph
Explanation:
Distance is proportional to time when velocity is constant. It is not constant in the case of stop-and-go traffic, a baseball*, or a slowing car. The speed of the person biking is given as a constant 12 mph, so that person is traveling a distance proportional to time.
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Additional comment
* It depends. The usual assumption is that horizontal speed is constant, in which case the distance from the hitter along the ground is proportional to time. If you are modeling the real world, the ball slows due to air resistance, so distance is not proportional to time.
If you are concerned with the actual distance the baseball travels through the air, the ball's speed slows as it gains height, then increases again as it falls to the ground. The speed is not constant during any part of that travel.