用一个成语概括本文《黔之驴》的意思，这个成语是____ 。它的含义是 ____ 。

Last Sunday the Greens went to climb the hill. When they got home, it was very late. Mr. Green opened the front door and they both went into the house. Because it was too late, Mrs. Green turned on the light. Just they wanted to come into the bedroom, Mrs. Green suddenly said, “ Listen, there is someone in our bedroom.” They stood quietly outside the door. They could hear some voices in the room.
“There are two men in it. They are talking.” So Mr. Green began to shout. “Who is there?” But no one answered. Mr. Green opened the door quickly and turned on the light. There was no one there. Mr. Green found something and laughed. The radio was still on! “I forgot to turn it off this morning.” Mr. Green said.

生活离不开化学，处处留心皆学问，请用下列物质的序号填空．

a．甲烷   b．熟石灰   c．干冰    d．活性炭

（1）可用于改良酸性土壤的是____ ；

（2）新农村建设中，许多家庭建了沼气池，沼气的主要成分是____ ；

（3）可用于净水的是____ ；

（4）可用于人工降雨的是____ ．