If $\frac{2}{3}$ of the seats at a football stadium were filled in the beginning of the game, and at halftime 1000 spectators left, leaving $\frac{3}{7}$ of the seats filled, what is the total number of seats in the stadium ?

Question

$\begin{array}{1 1} (a)\; 3700 \\ (b)\; 4000 \\ (c)\; 4200 \\ (d)\; 4500 \end{array}$