Rockets are able to accelerate in space because: a. they have excess fuel to burn from their initial take-off c. the force of gravity propels them b. they push exhaust in the same direction as they wish to accelerate d. they push exhaust in a direction opposite to the direction that they wish to accelerate

d. they push exhaust in a direction opposite to the direction that they wish to accelerate

Explanation:

This is a consequence of Newton's third law (action-reaction law), which states that

"When an object A exerts a force on an object B (action), object B exerts and equal and opposite force on object A (reaction)"

In this case, we can think the rocket as object A and the exhaust as object B. The rocket pushes the exhaust in a certain direction,applying a force on it; as a result of Newton's third law, the exhaust exerts the same force but on opposite direction on the rocket. Therefore, the rocket is pushed by the exhaust in the opposite direction, and therefore it accelerates because of this force.

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d) physical means can be used to separate the components of air.