I think the answer is "each other" for all of them?
Answer:
D
Explanation:
It has the most specific details of who and what.
A doesn't say specifically where. B and C both don't say specifically who
This makes D the most concise
It is whom. You can only use who if you can replace is with he or she.
The most suitable topic would be Medical testing for animals
An appropriate topic for 4-5 page research paper should specific, be at least measurable in some ways and had a lot of open sources.
Animal rights and activism is too broad of a subject and When john Muir started the sierra club is too specific and will be hard to find a lead for.
