$(5$ points$)$ At Center #$3,$ the evaluators are considering ways to improve the study during the second year of the foundation grant. Which of the following changes to the study design would most improve the researchers$$ ability to understand the relationship between hours of training and RII? Select one:

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a$.$ Eliminate the two outliers who have $35+$ hours of training.

b$.$ Add more youth with $35+$ hours of training so that they are approximately equal in number to the youth with about $15$ hours of training.

c$.$ Encourage more diversity in the number of hours of training so that they are spread across the range of approximately $15$ to $35$ hours of training.