AHSME 1950 Problem 27

A car travels $120$ miles from $A$ to $B$ at $30$ miles per hour but returns the same distance at $40$ miles per hour. The average speed for the round trip is closest to: $\textbf{(A)}\ 33\text{ mph} \textbf{(B)}\ 34\text{ mph} \textbf{(C)}\ 35\text{ mph} \textbf{(D)}\ 36\text{ mph} \textbf{(E)}\ 37\text{ mph}$