What is a main idea of the passage? A.
Things that at first seem crazy or ridiculous may turn out to be sensible.
B.
Some people enjoy adventure, but most prefer the safety of home.
C.
Things that appear to be mild and harmless may actually be threatening.
D.
Some places are worth exploring, but most are better off left alone.

Going up that river was like traveling back to the earliest beginnings of the world, when vegetation rioted on the earth and the big trees were kings. An empty stream, a great silence, an impenetrable forest. The air was warm, thick, heavy, sluggish, and there was no joy in the brilliance of sunshine. The long stretches of the waterway ran on, deserted, into the gloom of overshadowed distances, and on silvery sandbanks hippos and alligators sunned themselves side by side. The broadening waters flowed through a mob of wooded islands; you lost your way on that river as you would in a desert, and butted all day long against shoals, trying to find the channel, till you thought yourself bewitched and cut off for ever from everything you had known once—somewhere—far away—in another existence perhaps. There were moments when one's past came back to one, as it will sometimes when you have not a moment to spare to yourself, but it came in the shape of an unrestful and noisy dream, remembered with wonder amongst the overwhelming realities of this strange world of plants, and water, and silence. And this stillness of life did not in the least resemble a peace. It was the stillness of a relentless force brooding over a mysterious intention. It looked at you with a vengeful aspect. I got used to it after a while. I did not see it anymore. I had no time. I had to keep guessing at the channel. I had to discern, mostly by instinct, the signs of hidden banks. I watched for sunken stones. I was learning to clap my teeth smartly before my heart flew out, when I shaved by just barely some infernal sly old snag that would have ripped the life out of the tin-pot steamboat I drove and drowned all the passengers. I had to keep a look-out for the signs of dead wood that we could cut up in the night for the next day's steaming. When you have to attend to things of that sort, to the mere incidents of the surface, the reality—the reality, I tell you—fades. The inner truth is hidden—luckily, luckily. But I felt it all the same. I felt often its mysterious stillness watching me.