AMC 12 2018b Problem 2

Sam drove $96$ miles in $90$ minutes. His average speed during the first $30$ minutes was $60$ mph (miles per hour), and his average speed during the second $30$ minutes was $65$ mph. What was his average speed, in mph, during the last $30$ minutes?

$\textbf{(A) } 64 \textbf{(B) } 65 \textbf{(C) } 66 \textbf{(D) } 67 \textbf{(E) } 68$