从Ca、Na、C、H、O、N六种元素中选择适当的元素，按要求填空．

（1 ）用适当的数字和符号填空：

①两个钙离子____ ；②3个氨气分子____ ；

（2）写出符合下列要求的物质的化学式：

①造成“温室效应”的单质是____ ；

②一种氮肥是____ ；

③实验室常用的液体燃料是____ ；

④蒸馒头用的发酵粉中含有的盐是____ ．

一次函数y=kx+b的图象如图所示，则k、b的值为（   ）

读山区（林区）与平原（水稻产区）商业网点示意图，并回答下列问题：

    Long, long ago, there was a man. He lived in a village. One day, he went to a market and did some shopping. He suddenly found two boxes. Both the two boxes were very beautiful and the man loved them. So he bought the two boxes and took them home.

    He looked at the boxes. He didn't know how to use the boxes. He thought it over and decided to do something special. He decided to put all of his happiness in the yellow box and all of his sadness in the black box. He thought that it was funny to do that.

    A few days later, the yellow box was very heavy, but the black box was still light.

    He didn't know why. He decided to open the two boxes and found out the truth. He opened the yellow box and found nothing. Then he opened the black box and saw a hole (洞)at the bottom(底).