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 ).

one state would change   like louisiana to texas. thats a change

explanation:

- "the choice that is not an indicator of a chemical change is "state of matter changes". more common than not, chemical reactions produce energy in the form of light or heat. along with energy, they also produce a new substance called the product that could be in any state of matter (solid, gas, or liquid)."