$7)$Suppose a monopoly has a total cost function TC $=100$Q $+5$Q$2$ and a marginal cost function MC $=100+10$Q$.$ The market demand is given by the function P $=100010$Q$.$ To maximize profit, what Q should this firm choose?

$$

a$)$

$25$

$$

b$)$

$45$

$$

c$)$

$50$

$$

d$)$

$30$