Gravitation WarmUp Question 3

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The Acme engineering company has been hired by a wealthy person to investigate the feasibility of a train that travels in a tunnel through the center of the Earth. The Acme engineers have programmed a Physlet  simulation so that you can better take a look. The simulation shows the train (modeled by Acme as a spherical black ball) moving through the tunnel, and four possible graphs (one for each simulation) to represent the train's acceleration versus time.

Which of the four Physlet animations correctly shows the train's acceleration versus time?