Let $C(x)$ represent the cost and $R(x)$ the revenue, in dollars, associated with producing and selling $x$ units of Dr$.$ Little's Winning at Love $-$ A Strategy Guide to the Game of Tennis. If C$\text{'}(90)=63$ and $R^{\text{'}}(90)=63,$ which one of the following statements is true?

Producing and selling the $91$st item will decrease total profits.

Producing and selling the $91$ st item will increase total profits.

Producing and selling the $91$ st item will have little to no effect on total profits.

Since marginal cost equals marginal revenue, total profits are negative when $90$ items have been sold.

Since marginal cost equals marginal revenue, total profits are positive when $90$ items have been sold.