In a covalent bond, a.) Atoms share one or more pairs of electrons. b.) Adjacent atoms share a single electron. c.) An atom gains one or more electrons from another atom to gain stability. d.) An atom loses electrons to another to gain stability.

A) Atoms share one or more pairs of electrons.

Explanation:

Covalent bonding occurs when pairs of electrons are shared by atoms. Atoms will covalently bond with other atoms in order to gain more stability.

your answer is "the 1950's"

"a.) well–developed vascular system."