Fossil Fuels are considered nonrenewable resources because they ... a. burn so quickly

b. are in such high demand

c. take millions of years to form

d. pollute the air

It is choice "c" because a nonrenewable source is one that is consumed more rapidly than it is produced.

It is NOT "a" because while they may burn quickly, that isn't what makes it a nonrenewable source. The problem or factor that makes it nonrenewable source is it's inability to produce more faster than it is consumed.