How do plants contribute to the carbon cycle?
a. When plants respire, they release $\mathrm{CO}_{2}$ into the atmosphere
b. When plants photosynthesize, they consume $\mathrm{CO}_{2}$ from the atmosphere.
c. When plants photosynthesize, they provide oxygen to heterotrophs.
d. When plants emigrate, they transport carbon molecules between ecosystems.
e. Both a and b are correct.