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The African Sahara has mesmerized the world for centuries.At 3.3 million square miles, this desert is the largest in the world.Caravans in search of gold, ivory, salt and slaves made the Sahara the open gate to Africa.

Explanation:

"African" because the word is an adjective, describing a noun. "largest" because a superlative adjective is required. "open" because it remained open and didn't 'close' at the time being described.

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