Teacher:In this task, you're given a question, a context passage, and four options which are terms from the passage. After reading a passage, you will get a brief understanding of the terms. Your job is to determine by searching and reading further information of which term you can answer the question. Indicate your choice as 'a', 'b', 'c', or 'd'. If you think more than one option is plausible, choose the more probable option to help you answer the question.
Teacher: Now, understand the problem? Solve this instance: Question: Where was McMahon's post-doctoral mentor from? Passage:McMahon contributed to the understanding of retinal neurophysiology alongside his post-doctoral mentor, John E. Dowling. His early research focused on ion channels that mediate transmission at electrical and glutamatergic synapses and the modulatory effects of dopamine and nitric oxide on retinal synapse networks. Through studies with zebrafish he discovered that the neurotransmitter dopamine decreases the electrical coupling within horizontal cells. Further research showed that it was the increase of cAMP within the cell resulting from dopamine binding to AMPA receptor that led to this decrease in coupling. McMahon and his colleagues also demonstrated that exogenous nitric oxide and zinc can modulate AMPA receptor mediated synaptic transmission at gap junctions in hybrid bass retinal neurons.
 Links: a. Nitric oxide b. John E. Dowling c. Zinc d. John E. Dowling
Student:
b