When an organism has died, the ratio of c-14 to c-12 over time.
a. is constant
b. increases
c. decreases

The answer is C. decreases.

C-14 and C-12 are carbon isotopes used to measure the approximate age of organic remains. C-12 is stable, and it does not break down into other elements over time. C-14 is not stable. It breaks down into N-14.

Organisms can incorporate C-14 into their bodies during the life. But when the die, C-14 is not incorporated anymore. After organisms' death, C-12 will remain stable, but C-14 will start to breaks down into N-14. So, it will be less C-14 and C-12 will remain the same. The ratio of C-14 to C-12 must decrease over time.