19. Plastic bags are useful for holding many kinds of food ______ their cleanness, toughness and low cost.

A. by virtue of B. at sight of C. by means of D. by way of

20. — How’s the young man?

— ______.

A. He is twenty B. He’s a doctor

C. He’s much better D. He’s David

21. A museum in the city center ______ next year.

A. will be built B. will built

C. was built D. built

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