How does carbon get from the atmosphere to the geosphere? And how does the carbon from the carbon from the geosphere get to the atmosphere?

Explanation:

"Carbon enters the geosphere through the biosphere when dead organic matter (such as peat or marine algae) becomes incorporated into fossil fuels like coal and organic-matter-rich oil and gas source rocks, and when shells of calcium carbonate become limestone through the process of sedimentation."

Action potentials from different sensory modalities are carried on different nerve tracts.

Explanation:

Action potential in the cells occur upon the stimulation of the signal. The action potential results in the change in the membrane potential of the cell and responsible for the transmission of nerve impulse.

Different sensory receptors are present on brain to sense the different action potentials of the body. The different sensory modalities are used for the different nerve tracts and information flows occur in the body in the specific area.

Thus, the correct option is (a).