Which BEST describes the difference between carbon-12 and carbon-14

Carbon-12 and carbon-14 are two isotopes of the element carbon. The difference between carbon-12 and carbon-14 is the number of neutrons in each of their atoms. ... Atoms of carbon-12 have 6 neutrons, while atoms of carbon-14 contain 8 neutrons.

the solute can move "downhill," from regions of higher to lower concentration, relying on the specificity of the protein carrier to pass through the membrane. this process is called passive transport or facilitated diffusion, and does not require energy.