If $S_1=\{1,2,...................20\}$, $S_2=\{a,b,c,d,e\}$, $S_3=\{a,c,e,f\}$, then no. of elements of $(S_1\times S_2)\cap (S_1\times S_3)$ is ?
$\begin{array}{1 1} 40 \\ 60 \\ 80 \\ 100 \end{array}$