Why should we learn to swim?
Firstly, swimming is a good exercise for our health. Indeed doctors say it is the best exercise, because in swimming all the muscles of the body are used.
Secondly, swimming gives great pleasure. How nice it is, on a hot dusty day in summer, to take off one’s clothes and dive into the cool water of a rive or the sea, and to swim and float until one feels enough.
Thirdly, swimming may sometimes mean all the different between life and death, we never know when we may be in danger from water. We got on a voyage, and the ship in wrecked; we go for a pleasure sail on a lake and the boat turns over; we slip as we are walking along the bank of a river, and half in the water. If we can swim, we have chance for saving our own lives, but if we cannot swim, we are pretty certain to be drowned. So for our own safety, we should learn to swim.
Lastly, if we can swim, we may be able to save other people from drowning. How sad it is to see a friend drowning before our eyes while we cannot help just because we cannot swim.
C（气）＋D（气），当下列物理量不再变化时，不能表明反应已达平衡的是（ ）
