21. when does a nuclear fission reaction occur? a. when a nucleus is cut in two using a really sharp knife. b. when two nuclei collide and combine to one. c. when a large nucleus spontaneously is divided for no apparent reason. d. when the electric forces within the nucleus are stronger than the nuclear forces.

C

Explanation:

It's not A because you can't use a knife to cut an atom nucleus.  Its size is not perceptible to the eye.

It's not B because when two nuclei collide and combine into one, it's call fusion.

It's not D because you can't have stronger forces than the ones holding the nucleus together.

The correct answer is C.

To fission, the nucleus needs to have at least two things: critical mass (only big ones can fission spontaneously) and instability (incorporate a neutron ).