单项选择题

The investigation group tried in ()to discover what caused the failure of the engine.

A. diligence
B. vain
C. vigor
D. zeal

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Swedish is the () language of most Swedes.

A. natural
B. native
C. home
D. mother
单项选择题
We must () on water during the dry season.

A. economical
B. economize
C. economy
D. economically
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