The correct answer is - a. was a sign of danger.
Once the people saw that the ocean waters are receding and were living vast space without water behind them, they knew that something big and very dangerous will happen. And in fact it did. The water that was sucked in in the place were there was a crack on the ocean floor, got shot back under big pressure and it had very big speed, as well as having waves that were destroying anything on their way.
It had a gravitational force that acts so that it pulls Earth in an orbital, which explains temperature variations throughout the years.