Nerve stimulation studies show decreased skeletal muscle responsiveness on repeated motor neuron stimulation. A 39-year-old woman is diagnosed with myasthenia gravis and is treated with the drug pyridostigmine, which inhibits acetylcholine esterase. After treatment, she reports a return of muscle strength. These findings suggest that... a) the nicotinic receptors at her NMJ were overly stimulated b) the muscarinic receptors at her NMJ were insufficiently stimulated c) the nicotinic receptors at her NMJ were insufficiently stimulated d) the muscarinic receptors at her NMJ were overly stimulated