Why is critical mass important for a fission chain reaction?

Question

-keeps neutrons from escaping the sample.
-keeps neutrons from being absorbed by other isotopes.
-It allows neutrons to be absorbed by other fissionable nuclei
-it provides enough fuel to make enough energy.

Answer 1

-It allows neutrons to be absorbed by other fissionable nuclei

Answer 2

I disagree with the answer by anonymous. The correct answer, I think, is that it keep neutrons from escaping the sample. Too little sample (i.e., less than the critical mass) means the neutrons can get outside the sample without entering another fissionable atom so there is no chain reaction.