Answer: the Sahara Desert begins at the Atlantic Ocean and extends to the Red Sea.
The Sahara Desert begins at the Atlantic Ocean and extends to the Red Sea.
Explanation: As a general rule proper nouns take capital letters. Generally, articles are not capitalized but if they start a sentence after a writing or after a period, articles can be capitalized. in the second sentence I capitalized the first "the" because it goes after a period.
