$1\mathrm{.}25$ A research study showed that $7$ percent of $$A$$ students smoke, while nearly $50$ percent of $$D$$ students do$.$

$($a$)$ List in rank order six factors that you think affect grades. Is smoking on your list?

$($b$)$ If smoking is not a likely cause of poor grades, can you suggest reasons why these results were observed?

$($c$)$ Assuming these statistics are correct, would $$D$$ students who give up smoking improve their grades? Why or why not?