The answer is tighten and contract.
In order for the students to pull the rope their muscles will need to tighten and contract so they are able to continuously pull with all their strength and power. Relaxed or twitched muscles will result in release of the rope.
The spinal cord sends messages to the muscles when to contract ;so spinal cord would be your answer.
I would guess the answer is B. Options for physical activities are virtually endless.
<span>The correct answer is A. Air hose. That's because the air hose is elastic on its own and becomes firm when air goes through it. If you were carrying it by the hose, then if the gear malfunctioned and air started rushing through you wouldn't be able to control the device and it could hurt you or anyone near you as it would go rampant and you wouldn't be able to catch it.</span>