Thanks for posting. I hadn't thought of it before.
The quick answer to this is that they gather leaves to make boats. As a science major, I'm a little doubtful this would work. Those ants covered acres and acres and their size though relatively small, were huge compared to other ants. The surface tension of water with a leaf might be enough to accommodate 20 ants, but that was a spit in the bucket.
Further, this implies that the ants were discriminating enough to stop eating the vegetation (which is the central conflict of the story) and decide that they had to forestall their appetite so they had leaves to cross. Even if they were capable of such higher lever mental abilities, there likely were not enough leaves around to accomplish the crossing.
All of that just so I could answer A
