Our boat floated on between walls of forest. It was too thick for us to get a view of the land we were passing through, though we knew from the map that our river must be passing through chains of hills from time to time. Nowhere did we find a place where we could have landed: although the jungle did not actually spread right down into the river, banks of soft mud prevented us from going ashore. In any case, what would we have gained by landing? The country was full of snakes and other dangerous creatures and the jungle was so thick that one would be able to advance slowly, cutting one's way with knives the whole way. So we stayed in the boat, hoping that when we reached the sea, a friendly fisherman would pick us up and take us to civilization. As for water, there was a choice. We could drink the muddy river water, or die of thirst. We drank the water. Men who have just escaped from what appeared to be certain death lost all worries about such small things as diseases caused by dirty water. In fact, none of us suffered from any illness as a result. One day we passed another village, but fortunately nobody saw us. We did not wish to risk being taken prisoners a second time: we might not be so lucky as to escape in a stolen boat again. TOC \o "1-3" \h \z What they could see on the boat was only.
high walls chains of hills heavy woods vast land
Question: Our boat floated on between walls of forest. It was too thick for us to get a view of the land we were passing through, though we knew from the map that our river must be passing through chains of hills from time to time. Nowhere did we find a place where we could have landed: although the jungle did not actually spread right down into the river, banks of soft mud prevented us from going ashore. In any case, what would we have gained by landing? The country was full of snakes and other dangerous creatures and the jungle was so thick that one would be able to advance slowly, cutting one's way with knives the whole way. So we stayed in the boat, hoping that when we reached the sea, a friendly fisherman would pick us up and take us to civilization. As for water, there was a choice. We could drink the muddy river water, or die of thirst. We drank the water. Men who have just escaped from what appeared to be certain death lost all worries about such small things as diseases caused by dirty water. In fact, none of us suffered from any illness as a result. One day we passed another village, but fortunately nobody saw us. We did not wish to risk being taken prisoners a second time: we might not be so lucky as to escape in a stolen boat again. TOC \o "1-3" \h \z What they could see on the boat was only.
A. high walls
B. chains of hills
C. heavy woods
D. vast land
Answer: