I would say that there is no other living thing on earth that can react to medical testing like animals can, for example, testing it on plants wouldn't do us any good. but if you tested it on animals and it changed their hair purple or something then you gained new information that with out testing would have been unknown.
if you have options then I can help more, since it says "would most weaken"
For a variable to make since it has to have at least one other variable
Become is a example of a "linking verb." A linking verb is a verb that describes a object while connecting it to another noun or adjective. A example of a linking verb would be something like: The eggs smells rotten, or Jared has become a werewolf.
Hope this helps.
Perseverance, cleverness, focus, and determination are correct.
