Which statement best characterizes chemical reactions?

they always occur very quickly.
their rates vary significantly.
their rates are always slow.
they rarely occur in the natural environment.

Option (b) is the correct answer.

Explanation:

Chemical reactions are the reactions with the help of which rearrangement of molecular or ionic structure of a substance can be done.

The rate of a chemical reaction depends on various factors like nature of reactants, catalyst etc. Therefore some chemical reactions are fast, some are slow or some may be too slow.

Thus, we can conclude that their rates vary significantly.