Punctuate the sentences by adding commas or periods. Do not add any words. Capitalize where necessary.

11. He slapped his desk in disgust he had failed another examination and had ruined his chances for a passing grade in the course.

12. I struggled to keep my head above water I tried to yell for help but no sound came from my mouth.

13. The earthquake was devastating tall buildings crumbled and fell to the earth.

14. It was a wonderful picnic the children waded in the stream collected rocks and insects and flew kites the teenagers played an enthusiatic game of baseball the adults busied themselves preparing the food supervising the children and playing a game or two of volleyball.

15. Some people collect butterflies for a hobby these collectors capture them with a net and put them in a jar that has poison in it the dead butterflies are then mounted on a board.

16. Caterpillars eat plants and cause damage to some crops but adult butterflies feed principally on nectar from flowers and do not cause any harm.

17. The butterfly is a marvel it begins as an ugly caterpillar and turns into a work of art.

18. The sight of a butterfly floating from flower to flower on a warm sunny day brightens anyone's heart a butterfly is a charming and gentle creature.

19. When cold weather comes some butterflies travel great distances to reach tropical climates.

20. Butterflies are admired throughout the world because they are beautiful they can be found on very contient except Antarctica.
(Tư liệu tham khảo: Betty Schrampfer Azar . Understanding and Using English Grammar. 3rd ed.)

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