So we ran back , and then started walking again when we were near the entrance and the mosquitos lessened . We talked and watched the lake next to us . A train went by to our other side , slightly obscured by the trees , and we waved as it passed .

Available options:
[A]. None of the above choices ..
[B]. So they could leave the park ..
[C]. To get away from the mosquitos ..
[D]. To catch up with the train ..
Q: Why did they run back ?
[C].