Which sentence from the passage best supports the idea that Robinson was forced to adapt to his surroundings to keep time?
The matter of keeping track of time puzzled Robinson very much because it was getting more difficult every day to keep it in his memory. He must write down the days as they slip by, but where and how? He had neither pen, ink, nor paper. Should he mark every day with a colored stone on the smooth side of the huge rock wall within whose clefts he had dug out his cave? But the rain would wash off the record, and then he would lose all his bearings. Then he thought of the beach, but there, the wind and waves would also erase it.
He said to himself. I must find something on which to keep a record. I must also know when Sunday is, because I must rest one day in a week. Yes, I must find something on which to write..
Finally, he chose two trees standing near each other and then searched for a small, sharp stone, which he could make still sharper by striking it on another, and when he had got this pen ready he cut into the bark of one tree.
He made seven cuts in a row for the seven days in a week. The first cut was longer than the others to represent Sunday. At sunset every day, he made a new cut in the bark.
The other tree he called the month tree. On its stem, he was to cut a mark every time his week tree told him a month had passed, but he must be careful, for the months were not of equal length. He remembered his teacher had once said in school that the months could be counted on the knuckles and hollows of the hand, in such a way that the long and short months could be found easily, and he could tell, in this way, the number of days in each.