根据短文大意选择单词并用其正确形式填入空白处,使补全后的短文意思通顺,语法正确,结构完整. 

    Trees are useful in three important____  .The most important way is that they  provide man with food , wood and other products. Trees provide food not only for man, but also for animals.____trees, many animals could not live on the earth. It's not ____ for man to live on the earth, either.
    The second important way is that trees give us shade(树荫). On a hot summer day, people are eager to have a rest under the shade of a tree after____a long way. You can imagine how important the shade of a tree is to man and animals.
    The third important way is that trees help to____ droughts and flood. However, in many parts of the world, man hasn't realized it. He has cut trees down in large numbers. In the end, he finds that he has lost the best friends he had.

有关声的利用可分为两类：____和____。外科医生可以用超声波除去人体内的结石，是利用了声波能传递____。中国传统中医用望、闻、问、切的方法检查病人，其中“闻”就是____的意思，它是利用声波能传递____。

用符号和数字表示：

已知$a^2+b^2=1,b^2+c^2=2,c^2+a^2=2$，则$ab+bc+ca$的最小值为（   ）