Answer:
a. A car stopped at a stop sign.
d. A person running at 5 meters/second along a curving path
Explanation:
There is acceleration in all cases velocity changes. In options b and c the speed is constant, then there is no acceleration. In option a, the car was moving and then stop, therefore, its speed decrease to zero, and that represent an acceleration. In option d, the magnitude of the velocity doesn't change, nevertheless its direction changes along the curving path (remember that velocity is a vector quantity), therefore, the person receive centripetal acceleration
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