What dose it mean for a molecule to diffuse down a concentration gradient?

During convection, less dense fluid rises and denser fluid sinks.

Which of the following must be true for a fossil organism to be useful as an index fossil? a. the fossil organism must be widespread geographically. b. the fossil organism must be abundant. c. the fossil organism must be limited to a short span of geologic time. d. all of the above.

Bio molecules are essential to all living things. which biomolecule is “d” from the table