AHSME 1961 Problem 17

In the base ten number system the number $526$ means $5 \times 10^2+2 \times 10 + 6$. In the Land of Mathesis, however, numbers are written in the base $r$. Jones purchases an automobile there for $440$ monetary units (abbreviated m.u). He gives the salesman a $1000$ m.u bill, and receives, in change, $340$ m.u. The base $r$ is: $\textbf{(A)}\ 2 \textbf{(B)}\ 5 \textbf{(C)}\ 7 \textbf{(D)}\ 8 \textbf{(E)}\ 12$