The great river Nile flows gently in its course through the hot plains in the first half of the year but later on when the melting snows and the rains on the mountains far to the south swell its tributaries, the Nile overflows. It spreads rich, muddy soil from Ethiopia over its valley and forms deep stretches of green, fertile lands along its banks. The settlers found that in the soft rich earth wheat and other crops could be planted, even without the use of the plough, and they began to make many settlements of farmers. 
In these early times they did not of course understand why the river overflowed each year. But they knew that their crops and, therefore, their lives, depended upon its magic floods, and they explained the miracle as the work of gods. 
But there came some years when there was a "bad Nile". Sometimes the floods were not full and did not bring enough soil; the crops were poor and the people starved. At other times the waters were so great that they destroyed houses and villages, and drowned men and beasts. It took perhaps many centuries before the farmers learned how to control the Nile waters. 
Wise men among them watching the position of the stars year by year found that they could predict when the annual rising of the Nile would come. Thus, they began to learn about the scientific study of the sun, earth, moon, and stars and could make a calendar of the years. They also learnt how to measure out the land so that it could be divided fairly again after the boundaries of the farms had been washed away by great floods. In this way, there came about ancient knowledge of engineering and of geometry.
7. The settlers began farming in the Nile valley because ______. 

A  they discovered that their crops could grow well in the soil there 
B they did not realize that the river would overflow sometimes 
C it was not necessary for them to use the plough when cultivating the soil 
D  they realized that their crops depended on the floods