Adrug will prevent an action potential if it a. lowers the threshold of the membrane b. blocks the movement of potassium across the membrane c. blocks the movement of sodium across the membrane d. increases the movement of sodium across the membrane

It prevent an action potential if it blocks the movement of sodium across the membrane.   The primary feature of a neuron that prevents the action potential from traveling back from where it just passed is the absolute refractory period. It holds an action potential if it blocks the movement of sodium across the membrane.   

good one: produce hormones that plants grow.

bad one: the problem with organic matter is that while plant nutrients are needed for crop growth, they can also stimulate excessive growth of the wrong plants – mainly, algae.