I think the correct answer from the choices listed above is the last option. To determine the answer, we substitute 3 and 10 to each choice and see whether the answer would equate to 0 for both values of x. We do as follows:
The distance that the train traveled in 30s is the difference of the total length of the tunnel and the train (2000m). This means it travels 2000m/30s. This ratio is the same as 240000/3600s or 240km/1hr.