阅读下面短文，从每题所给的A、B、C和D四个选项中，选出最佳选项。
One day, a boy was playing in a garden. He was running after butterflies and trying to catch them. He was very1to see the colorful flowers, too. Then he saw a frog skipping and2As he ran behind the frog, it went into the pond. A cow was eating grass in the garden. He saw how the cow was3grass. A fly then came there and buzzed over the 4It swung its tail to make the fly move away. The child clapped and 5
The boy was enjoying the beauty of nature. He said,＂Nature is6 Its beauty attracts everyone.＂
When he saw a squirrel, he ran after it and7near the bed of roses. He tried to pick some of the roses, but a thorn 8his finger. It was very painful. The boy started9and ran to his mother who was in the10
His mother asked him,＂What is the matter?＂The boy said, ＂The thorn of the rose 11 my finger while I was picking it and now I feel12 pain.＂ His mother said,＂That was your13You should not have tried to pick the flower. The flowers are meant to be14and not to be picked. Besides, natural beauty should never be 15We should help to protect nature.＂The son said,＂I will 16remember what you have just told me, Mom.＂The mother said,＂Let me know what you have 17today.＂ Her son explained,＂We should protect 18living beings, whether animals or19 We should never harm them. We should not harm nature.20we do, it may harm us in the long run, too.＂
”和“ 
”表示两种不同原子。分析回答，为什么物质发生化学反应都遵循质量守恒定律？