Which of the following are sources of low-level radiation? a) medical x-rays b) nuclear weapons testing c) buring of fossil fuels d) all of the above e) a and c only

The answer to this question would be E)a and c only

Nuclear weapon testing definitely gives a high-level radiation.  Since the explosive power should be big, the amount of nuclear waste will also be big and it will give a high intensity of radiation. So option B and D will be out. 
Medical X-ray will give low-level radiation. It will be much lower than CT-scan or any imaging method that using radiation. Burning fossil fuels also produce some radiation.

16- counting by fours

Ibelieve it is b. proton.hope my answer has you : )