The answer is “of”
The process of selecting candidates is a difficult one.
C. How long does it take to travel from the Earth to the moon?
This is the weakest research question because it can be easily answered. According to a 2 second Google search, it takes about 3 days for a spacecraft to reach the moon from Earth. This question does not allow for a lot of research for development into an essay. Options A, B and D all have a variety of possibilities that would allow for a multi-paragraph essay with significant topic development.
