$20.)$ Another observer is standing at the train station $($Point D$)$ as Train B passes by at a distance of $d=7,000m$ away. $($See the diagram.$)$ At that instant, what is the minimum distance, $L,$ that Train A and Train B could be separated by in order to have destructive interference at the location of the observer? Assume Train A blows also blows its horn emitting a frequency of $500$ Hz $.$ Give an answer in meters.

$($A$)51\mathrm{.}6$

$($B$)98\mathrm{.}7$

$($C$)69\mathrm{.}3$

$($D$)87\mathrm{.}9$

$($E$)101$

Constants

Gas constant: $R=8\mathrm{.}31\frac{(J)}{(mol)*(K)}$

Speed of sound in air: $v=343\frac{m}{s}=1,235k\frac{m}{h}$