Đọc kỹ đoạn văn sau và chọn phương án đúng (A hoặc B, C, D) cho mỗi câu từ 66 đến 70

 The fourth letter of the Greek alphabet was named delta. Its sign was a triangle – a three – sided figure. Today the word delta means the triangular group of islands at the mouth of a river.

 Delta are formed by the rivers themselves. Large rivers may carry sand and fine soil as they sweep towards their mouths. Near their outlets, rivers usually slow, and can less easily carry their burdens. Sand and silt are dropped when rivers enter a larger body of water. Not all rivers form delta. Ocean currents may be strong enough to sweep the river water away before sand and silt are deposited. Or the deposits may themselves be swept away.

 The fertile soil of the Nile delta has made it famous. Mans history in this delta dates back for thousands of years.

A. a group of islands   B. the Third letter of the alphabet 

C. a kind of soil  D. the fourth letter of the alphabet