Given a scientific passage and an answer, generate a question for the given answer.

Q: Passage: boundary between air masses that is stalled in one place. Answer: aids

A: Opportunistic diseases are usually the direct cause of death of people with what disease?
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Q: Passage: The wide range of movement allowed by synovial joints produces different types of movements. Angular movements are produced when the angle between the bones of a joint changes. Flexion, or bending, occurs when the angle between the bones decreases. Moving the forearm upward at the elbow is an example of flexion. Extension is the opposite of flexion in that the angle between the bones of a joint increases. Rotational movement is the movement of a bone as it rotates around its own longitudinal axis. Movement of the head as in saying "no" is an example of rotation. Answer: angular

A: What type of movements are produced when the angle between the bones of a joint changes?
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Q: Passage: Torque is the turning effectiveness of a force. In this case, because F is perpendicular to r , torque is simply we multiply both sides of the equation above by r , we get torque on the left-hand side. That is, rF = mr 2. Answer: torque

A:
What is the turning effectiveness of a force?
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