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Should the word role in the following sentence be plural (roles) instead? Please explain why or why not.

"He had to take on the role of both father and mother."

The above sentence is an example from Merriam-Webster's Dictionary.

How about the below sentence?

"What is the role of weight and engine size in a car's performance?"

Should it be "are the roles"? Please explain.

Thank you.
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perplexedHow about the below sentence?

"What is the role of weight and engine size in a car's performance?"

Should it be "are the roles"? Please explain.
You could use either. Which you use depends on whether you want to emphasize the two characteristics in question or the overall performance of the car. Although I think you will find that most native speakers will use "is the role" in this situation.
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Thank you for the response. Does your explaination also applies to the first example of my question?