已知复数z=（m²-4）+（m²-m-6）i是纯虚数，则实数m=（　　）

A．±2

B．-2

C．2

D．3

B

Uncle Li and Uncle Wang are good friends. They live next to each other and their farms are both at the foot of the mountain. So they can help each other. But neither of them likes to use his head. They’re both poor though they work hard. Most villagers have built new houses, but they still live in the low and broken houses. They never find out why.

Once Uncle Li went to the town to buy some medicine for his wife. In the town he heard the apples in a city were expensive. He told Uncle Wang about it as soon as he went back. They decided to carry some apples to the city. They borrowed some money from their friends and bought nearly 1000 kilograms of apples in the villages and carried them to the city on a tractor. Bad luck! A lot of apples has already been carried there when they arrived. A few days later they had to sell them at a low price. They felt unhappy and returned to their village.

"I can’t understand why we sustained (蒙受) losses in business while others always profit (赢利)" Uncle Li asked one day.

"The tractor was too small " Uncle Wang said without thinking. "We’ll carry more apples on a truck next time !"

"I agree!" said Uncle Li. "How foolish we were!\

the two farmers had to sell their apples at a low price because（）.

A．theirs weren’t as good as the others’

B．theirs were much less than the others’

C．a lot of apples had been already carried to the city

D．they forgot to carry them on a truck