题目内容

The Grand Canyon in north-western ___ ____ is one of nature's most impeessive sights.

A. Utah
B. Arizona
C. Nevada
D. Idaho

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The United States went to war with ___ ____ in 1812, the last war fought between these two countries.

A. Britain
B. France
C. Spain
D. Mexico

In 1932, in the depth of the depression, the American people chose ___ ____ as their next president who promised a "new deal" to get America out of depression.

A. Theodore
B. Franklin Roosevelt
C. Woodrow Wilson
D. Herbert Hoover

如何理解群速度和相速度?

在什么情况下,群速度才有意义?

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