A、He got exhausted in trying to catch up with the boat. B、It was his last trip. C、The boat slowed down. D、He had not been so goo

Sunday, 2nd November, will remain in my memory because I did a very stupid thing. For some days my health had not been so good. The change of food and the constant humidity had caused a number of painful spots to form on my skin. I hoped to prevent them forming scabs by resting my weight on a little air cushion, the only one I had. Some clumsy movement must have knocked it overboard, a fact I only realized when I saw it floating a couple of hundred meters or so astern. I lowered the sail, put out the sea anchor and dived in to fetch it. I am strong swimmer and reached the cushion in a few minutes. Imagine my horror, when I turned around, to see the boat sailing off without me, too fast for me to be able to catch it. The sea anchor, normally shaped like a parachute, had become entangled and was no longer stopping the drift. It was quite clear that I would become exhausted long before I could overhaul it.
    When I was training to swim the English Channel in 1951, in top physical condition, I once swam for twenty-one hours. Weakened as I was, I could not possibly have equalled the feat. I abandoned the cushion to its fate and concentrated on the fastest swim of my life. I managed to cut down the distance a little, but then had difficulty in even maintaining it. Suddenly I saw the boat slow down. I caught it up and just managed to hoist myself on board. By a miracle the cords of the sea anchor had disentangled themselves just in time. I was exhausted and swore it was the last dip I would take on the journey.
Questions 19 to 21 are based on the passage you have just heard.
19. What was the "very stupid thing" the writer did?
20. Why did the writer finally catch up with the boat?
21. Why did the writer swear it was the last dip he would take on this journey?