题目

   Thirty years ago, Lake Ponkapog in Hartwell, New Jersey, was full of life. Many birds and animals lived beside the water, which was full of fish. The lake water is polluted . It is in a color of dirty brown, and it is filled with strange plants. How did this happen? First,  we must think about how water gets into Lake Ponkapog. When it rains, water comes into the lake from all around. In the past, there were forests all around Lake Ponkapog, so the rainwater was clean. Now there are many homes around the lake. People often use chemicals in their gardens. They use other chemicals ins用字母表示下面的运算定律． 乘法分配律________________________________