Question

what happens when the amount of fissionable material exceeds the critical mass?
the chain reaction accelerates as more neutrons are generated with each round of reaction.
the chain reaction is not sustained and eventually stops.
the density of the reacting particles decreases, slowing the reaction.
the amount of material needed to establish a new critical mass increases.

Explanation:

When the amount of fissionable material exceeds the critical mass, more neutrons are available to cause further fissions. This leads to an accelerating chain - reaction as each fission event generates more neutrons for subsequent reactions.

Answer:

The chain reaction accelerates as more neutrons are generated with each round of reaction.