]when the concentration of a substance reaches equilibrium on both sides of a cell membrane, it can continue moving in and out of the cell using

Active transport

Explanation:

Diffusion occurs when molecules move from an area of high concentration to a low concentration until equilibrium is achieved. This does not require energy because it is a spontaneous process. This also included osmosis when there’s a difference in the concentration of solutions across a semipermeable membrane, water molecules move from the low concentration the  solution to high concentration until the two solutions are isotonic to each other.

To move molecules against a concentration gradient, energy must be expended. In cell this is through the energy of ATP molecules.

a tetrad is the foursome during meiosis made by two homologous chromosomes that have each already replicated into a pair of sister chromatids.

the answer is d. chordata