Question 147·Hard·Boundaries
Neuroscientists have identified dozens of neurotransmitters in the human brain. Only three of these chemicals belong to a single molecular category _____ dopamine, norepinephrine, and serotonin—each of which plays a crucial role in regulating mood.
Which choice completes the text so that it conforms to the conventions of Standard English?
For boundary questions about punctuation and short connecting phrases, first identify how the ideas on each side of the blank relate: are they contrasting, listing, explaining, or joining equal clauses? Then recall which marks match each relationship (colon to introduce a list or explanation, semicolon to connect related independent clauses, commas for nonessential phrases, etc.). Quickly eliminate choices whose transition word does not fit the meaning (like however where there is no contrast) or whose punctuation pattern is not a standard, grammatical way to join those parts.
Hints
Look at what comes after the blank
Focus on the words immediately after the blank: dopamine, norepinephrine, and serotonin. What role do these words play in the sentence?
Identify the relationship between the ideas
Is the second part (the three named chemicals) contrasting with the first part, or is it explaining/illustrating the first part?
Think about punctuation for lists
What punctuation mark is commonly used after a complete thought to introduce a list of specific examples?
Question the transition word
Several options include the word however. Does however, which shows contrast, make sense for the relationship between the category and its three specific members?
Step-by-step Explanation
Understand the sentence structure
Read the whole sentence: we have a statement, Only three of these chemicals belong to a single molecular category ___ dopamine, norepinephrine, and serotonin—each of which plays a crucial role in regulating mood.
Before the blank, we are talking about a category; after the blank, we see a list of three specific chemicals in that category.
Decide the logical relationship between the parts
Ask what the second part (the list of 3 names) is doing. It is not contrasting with the first part; instead, it is giving specific examples or a label for that category.
So we need punctuation that introduces an explanation or a list, not a contrast word like however.
Recall the punctuation that introduces a list or explanation
In Standard English, a colon is used after a complete idea to introduce a list or explanation, especially when the words that follow clarify or name what came before.
Here, the phrase in the blank should smoothly connect the idea of the category to the specific examples that follow.
Check each answer choice against grammar and meaning
Choices that use however suggest contrast and also create punctuation errors (such as a semicolon plus however with a comma, or a comma followed by however and a colon). We also must avoid missing needed punctuation between clauses.
Only choice C, known as the monoamines:, correctly provides a descriptive phrase and ends with a colon to introduce the list of three neurotransmitters, so C is the correct answer.