Question

the two basic categories of neurotransmitter molecules are what?
right and left
back and front
up and down
small and large

Explanation:

Neurotransmitter molecules are classified by size. The two basic categories are small - molecule neurotransmitters and large - molecule neurotransmitters. The other options (right/left, back/front, up/down) are spatial directions and not relevant to neurotransmitter classification.

Answer:

small and large