Sound of an incoming automobile

Two trains separated by $2\text{ km}$ , travel with the same speed of $v = 90\text{ km/h}$ on the same straight track. A man sits at a point $A$ , located at a distnace of $1\text{ km}$ from the track. At the time when the trains are equidistant from point $A$ , they simultaneously emit a sound signal of the same frequency $f_0 = 500 \text{ Hz}$ .

Which of the following is the best approximation for the beat frequency hear by the man at $A$ .

Take the speed of the sound as $c = 350\text{ m/s}$ .