Swimming is very popular. People like swimming in summer because water makes people feel cool. If you like swimming but swim in a wrong place, it may not be safe. These years, a lot of people die when they enjoy themselves in water, and most of them are students. Some people are not careful when they are swimming. They often think they swim so well that nothing will happen to them in water. Summer is here again. If you go swimming, don't forget that better swimmers have died in water. They die because they are not careful, not because they cannot swim. So don't get into the water when you are alone. Don't get into the water if there is a " No swimming" sign. If you remember these, swimming will be safe. Some swimmers die in water because   _  .
they swim so well they are not strong enough they are not careful they can't swim well