Given scientific fact as input, generate the question from this fact such that it can be answered from the input.

Negative growth rate results when birth rates fall even lower than death rates.
What results when birth rates fall even lower than death rates?

A collapsing nebula leads to nuclear fusion process.
A collapsing nebula leads to what process?

Warm, humid climate allowed life to expand and diversify during the early cambrian period.

What allowed life to expand and diversify during the early cambrian period?