阅读短文，从方框中选择恰当的词并用其适当形式填空，使短文完整、通顺。每词限用一次，其中有两个词是多余的。
| as，sound，they；clever，send，imagine，heavy，hear，lucky ，know，if，follow |
Mr. and Mrs. Brown were farmers in South Australia．They lived there for many years．They had a kangaroo(袋鼠)in
____family．The kangaroo was called Lulu．Several years ago，Mr. Brown’s son gave Lulu to him
____a gift for his 60th birthday. Soon Mr. Brown and Lulu became good friends and Lulu often
____ him around the farm．
One day when Mr. Brown was working on his farm，a(n)
____tree branch(树枝)fell on top of top of him suddenly(突然)．Mr. Brown fell to the ground(地面)and didn't
____what happened next．Lulu stood next to Mr. Brow’s body．She started barking(叫)and didn't leave Mr. Brown．“She
____like a dog．She barked and barked．And she didn't stop．”said Mrs. Brown．After
____Lulu's barks, Mrs. Brown quickly went to see what had happened．She saw Mr. Brown lying on the ground and
____him to the hospital right away．
____，Mr. Brown was saved．
“Lulu is great!”said Mr. Brown．“She saved me! She is a friendly and very
____ kangaroo．”After Mr. Brown left the hospital, he took Lulu everywhere he went．
2NO+O2在体积不变的密闭容器中反应，达到平衡状态的标志是