A cold air mass pushes under a warm air mass. whats the type of front it describes?

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lewis and clark were appointed by president thomas jefferson to explore the land western of the mississippi river after he bought it (for $2.50 an acre) from napoleon of france. they explored places like present day nebraska, missouri, and north dakota.they had interactions with many tribes and used teenage indian girl sacagawea as there interpreter on their journeys. it took them 2 years, from 1804 to 1806. they got back and gained many praise from the president. on their journey they sent him many presents such as plants, seeds, and even a live prairie dog.

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