The graph below shows the rate of change in revenue with respect to price for a certain product where the price, $p,$ is in dollars and $R^{\text{'}}(p)$ is in dollars per dollar increase in price. Use the graph to determine which of the following is a TRUE statement.

a$)$ The revenue is decreasing for prices less than $$40\mathrm{.}00$ and it is increasing for prices greater than $$40\mathrm{.}00.$

b$)$ The revenue is decreasing for prices less than $$80\mathrm{.}00$ and it is increasing for prices greater than $$80\mathrm{.}00.$

c$)$ The revenue is increasing for prices less than $$40\mathrm{.}00$ and it is decreasing for prices greater than $$40\mathrm{.}00.$

d$)$ The revenue is increasing for prices less than $$80\mathrm{.}00$ and it is decreasing for prices greater than $$80\mathrm{.}00.$