资料题：According to the passage, the reason why early people kept moving before they learned to grow their own grass is that（　）。

A long time ago men, women, and children lived in small groups.There weren't any countries or big cities then.There weren't even any small towns.People lived in or near caves with their families.These early people moved from place to place looking for food.Sometimes they hunted animals.They also went fishing.Other times they picked fruit or wild plants to eat.They even made flour out of some of these plants.After a long time, people began to keep sheep and goats.Then they trained dogs to help take care of these animals.But families still had to move around a lot to find grass.They needed it to feed the sheep.After the sheep would eat up the grass, the people had move to another place to find more.As time went by, people began to raise cows.They learned to grow heir own grass, too.Then they didn't have to move around so much.So they began to build houses.Sometimes they made them out of rocks.Other houses were made of clay and mud mixed together.People kept working to have better lives.They wanted nicer homes.Some workers built tables and chairs.Others built different things from wood.More and more things were made.Then people began to trade, or give some of their goods to get others.As more and more things were traded, small towns grew into cities.