Artificial methods in common use include preventing the sperm from reaching the ovum (using condoms, diaphragms, etc), inhibiting ovulation (using oral contraceptive pills), preventing implantation (using intrauterine devices), killing the sperm (using spermicides)
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.
D.) Washed your hands first.
It is D.) because when you wash your hands first you're getting all the germs, dirt, or whatever is on your hands, off. Then you go back and put hand sanitizer to then in a sense "clean" it more. Plus who knows what is on that paper towel you just dried your hands with.
Exposure to the sun can cause cancer
