In this image, what must the students' muscles do to pull the rope? twitch and tighten relax and twitch tighten and contract contract and relax

The correct answer is: tighten and contract
This is is the correct answer because muscle fibers need to first reach an optimal length and tension ratio or "tighten" in order for the muscle to perform work. After the muscle has tightened, it can then "contract" which leads to a decrease in the angle of the joint through force. If the muscle being used to pull the rope is the bicep on the arm, then the angle between the forearm and upper arm will decrease, causing the rope to be pulled.

Lack of sleep can cause dis-functionality in your everyday routine and loss in weight

Your dog can sniff at or even eat a bit of catnip. the herb is edible and not harmful. just don't expect them to roll around like a cat would! your dog may scrutinize catnip by licking and smelling at it, or be totally indifferent to this stimulant.