Answer:
Molecule A is a simple sugar or amino acid, while molecule B is a starch or protein.
Explanation:
Molecule A, as a simple sugar or amino acid, consists of small molecules which can readily pass through the cell membrane, while Molecule B, as a starch or protein, consists of large molecules, thus, can't pass across the cell membrane.
This is the reason why the cell membrane is considered to be selectively permeable.
