阅读下列材料,在空白处填入适当的内容(不多于3个单词)或括号内单词的正确形式。
If you go out to the fields at night in spring or summer, you can hear frogs singing 13 (happy) here and there. It seems as if they were performing a field group singing.
The frog is a good and useful creature that benefits human beings. They can catch fast-moving 14 (insect). Each frog eats a large number of pests ( 害 虫 ) that are harmful to crops. This little creature is regarded 15 “the natural enemy of pests”.
But now frogs 16 (get) fewer and fewer. This is because they are killed and put 17 the table as a delicious dish by their chief enemy, human beings. It is a shameless and cruel act, isn’t it?
The cause that is 18 (responsibility) for the rapid reduction of frogs is that farmers use insect killer to kill pests and frogs get killed as a result of drinking poisoned water while 19 (eat) poison-killed insects.
Something must be done as soon as possible 20 (save) frogs. If we don’t punish those 21 sell and kill frogs to make money, then one day all of us 22 (punish) by nature for failing to keep them.