The answer is A, 'a mammal' because it goes with the copulative verb 'to be', i. e., 'is'.
Explanation:
Make an observation about a phenomenon (qualitative and/or quantitative)
Ask a research question.
Hypothesize a possible answer for your question.
Create a procedure to test your hypothesis.
Identify what you are testing.
Identify your control group/experimental group.
Answer:
a neurotransmitter binds to its receptor on a receiving cell
Explanation:
