阅读理解。阅读下列短文，从每题所给的四个选项(A、B、C和D)中，选出最佳选项。
Recognized as the largest desert in the world, the Sahara runs across North Africa, Covering over 3 million square miles from the Atlantic Coast to the Nile River Valley. Except for an occasional oasis(绿洲), this vast land is mainly made up of sand, stones, and worn out mountains. The burning sun causes daytime temperatures to rise to 70 degrees in the shade, and days without rain commonly last for years.
