which of the following is true about scientific theories?
1. new research is often performed to learn more about existing scientific theories.
2. new scientific theories can be formed to explain new phenomena.
3. scientific theories can be revised changed, or even rejected.
4. scientific theories are considered to be facts and cannot be modified

Explanation:

It certainly isn't the last one. A theory is close to a guess. It fits all the known facts, but there is usually something missing. If someone figures out what is missing, usually by experiment, then the theory become a fact.

The rest of them (1 2 an 3) can all be true. 3 is the least viable. You have to be able to come up with a counter example to reject a theory.

I'm not sure about 2. New phenomena can still be part of current theories. Reluctantly, I wouldn't choose 2 because

1 is the best answer of a very poor lot. Do not be surprised if you use it, that it turns out to be wrong.

c. heat from the poles back down towards the equator.