已知ab=4，若﹣2≤b≤﹣1，则a的取值范围是（　　）

根据材料内容，选择正确答案。

    Dick was born in a poor family. His father had a small boat and went fishing in the morning and sold the fish in the market in the afternoon. Then he bought some food for his family. When winter came, they were often hungry. One morning the hungry man fell into the river and wasn't found. Dick's mother left her three-year-old son without saying good-bye. His aunt had to look after him.

    Twenty years passed. Dick became a tall, strong man. He found work on a farm. He worked hard and wanted to get more money. He often went to see his aunt with some nice presents. The woman was very happy but one day she died in a traffic accident. The young man was very sad. After he buried her, he decided to buy a beautiful tombstone (墓碑)for her. He went to town and came in a shop, but all the tombstones were too expensive. He asked, "Do you sell an old tombstone, sir?" "Yes, we do, sir," answered the shopkeeper. "Is it as expensive as the new one?''

    "No, it's much cheaper," said the man. "But another name was engraved(刻) on it." "It doesn't matter," said Dick. "My aunt couldn't read."

氮化硅膜与二氧化硅膜相比较具有表面化学性能稳定等优点，故氮化硅膜可用于半导体工业．为生成氮化硅膜，可以用NH₃和SiH₄（硅烷）在一定条件下反应并在600℃的加热基板上生成氮化硅膜：

3SiH₄+4NH₃Si₃N₄+12H₂

列式计算。