Read the following passage and the unfinished sentences carefully. Then choose the best answer and circle your choice.
When springtime comes, everything begins to wake up. Many animals in the woods have been asleep all winter. Birds have been away. Under the snow, the ground has been hard and still.
Then, all at once, it is spring! The days begin to grow long. From the south the birds fly back, ready to build their nests. Animals wake up and start to look for food.
Trees grow new leaves. Plants begin to flower. Baby birds and animals are born. Farmers are busy from morning until night.
In many countries, people greet the spring with songs and dancing. Springtime is a happy time all over the world.
Birds fly south in winter because …………………
A. there are more trees for them there.
B. it is warmer in the south than it is in the north
C. they want a warmer place to lay eggs.
D. the days there are longer.
