阅读下面短文，按照句子结构的语法性和上下文连贯的要求，在空格处填入一个适当的词或使用括号中词语的正确形式（最多限填三个单词）填空。
An ant was walking by a river. She looked at the river and said to herself, “____ nice and cool this water looks! I must drink some of it.” But ____ she was drinking, she slipped(滑倒) into the river.
Luckily a bird was sitting in the tree. The bird quickly used his mouth ____(cut) a leaf from the tree. He dropped the leaf into the river near the ant.
The ant climbed onto the leaf, and the wind blew the leaf to the bank. And the ant ____(save).
“Thank you!”shouted the ant, ____ her low voice. But her voice was ____ low that the bird couldn't hear it.
After a few days, the bird was building his nest. Suddenly a bird catcher ____(careful) took out his nest(网). He quietly walked towards the branch where the bird was sitting.
The ant saw this, and ran quickly to bite the ____(man) leg. “Ouch” The pain in his leg made the man drop his nest. The bird heard the noise and ____ (fly) away.
The bird never knew how the little ant helped. ____ the ant was happy that she was able to do something for the bird.
的展开式中，各项系数之和为A,各项二项式系数之和为B,且
,则展开式中常数项的值为 （ ）