When motion is changing directions, acceleration is the rate at which , but when motion is not changing direction, acceleration may be expressed as the rate at which changes.

a. speed, time
b. velocity, speed
c. elapsed time, average speed

The best and most correct answer among the choices provided by your question is the second choice or letter B.

When motion IS changing directions, acceleration is the rate at which velocity changes, but when motion is NOT changing direction, acceleration may be expressed as the rate at which speed changes.

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