$9)$ Which of the following statements best summarizes the debate surrounding the nature of individual differences in working memory $($WM$)$ span?

A$.$ WM span is entirely determined by fixed information processing capacity and cannot be improved through training.

B$.$ WM span is solely determined by the use of effective strategies, and individuals with low WM span can significantly improve their performance through strategy training.

C$.$ Both information processing capacity and strategy use contribute to individual differences in WM span, and strategy training can further enhance performance in WM tasks.

D$.$ The relationship between WM span and strategy use is unclear, and more research is needed to determine the relative contributions of each factor.