The Table Shows the Relationship Between Output, Total Costs, and Total

The table shows the relationship between output, total costs, and total revenue for a pure monopoly. $\begin{array} { | c | c | c | } \hline \text { Output } & \text { TC } & \text { TR } \\\hline 50 & \$ 750 & \$ 1,000 \\\hline 60 & 800 & 1,100 \\\hline 70 & 950 & 1,250 \\\hline 80 & 1,200 & 1,450 \\\hline 90 & 1,300 & 1,500 \\\hline\end{array}$ Within which of the following ranges of output will the firm earn maximum economic profits?

A) 50 to 60 units
B) 60 to 70 units
C) 70 to 80 units
D) 80 to 90 units