Like China , Egypt was invaded by many foreigners during its history. Sometimes, invaders are absorbed into the Egyptian culture; but not always. Arabs conquered Egypt in AD 639, the time of the Tang Dynasty in China . They brought Islam and the Arabic language to Egypt which has remained. Today 90 percent of Egyptians are practicing Muslims. Arabic is the official language. The Nile River is the life-blood of Egypt . It flows through the capital city, Cairo, and north into the Mediterranean Sea( 地中海 ). At 6 600 kilometers, it's the longest river in the world. Most of the population live in the fertile ( 肥沃的 ) valley of the Nile. Its waters irrigate the land along its banks. The rest of the country is desert. The annual flooding of the Nile, today, as in ancient times, renews the land' s fertility and assures successful agriculture. The ancients built a great civilization on these banks. Today, millions from around the world come to see its wonders. Tourism is one of Egypt 's most important industries. Egypt is expecting more than 1. 5 million tourists this year, spending close to 1.2 billion US dollars, or 10 billion RMB. A tour up the Nile on a river boat is always fun, for tourists or natives.