Simple covalent molecules have low melting and boiling points because they have weak intermolecular forces (such as van der Waals forces, dipole-dipole interactions, or hydrogen bonds) between individual molecules. Relatively little energy is needed to overcome these weak forces during melting or boiling, which results in low melting and boiling points.

B. They have definite shapes: While covalent molecules do have definite shapes (explained by VSEPR theory), this is a characteristic of the individual molecule's structure and does not determine the energy required to separate the molecules from each other during a change of state.

C. They are three-dimensional structures: Similar to option B, the three-dimensional nature of covalent molecules describes their internal structure, not the forces between adjacent molecules that determine melting and boiling points.

D. They possess shared electron pairs: Covalent molecules possess shared electron pairs to form strong intramolecular covalent bonds (bonds within the molecule). These strong bonds require a large amount of energy to break, but they are not broken during the processes of melting or boiling.

Therefore,the correct answer is option A.