题目内容：

阅读下面材料，在空白处填入1个适当的单词或括号内单词的正确形式。

Wild elephants that destroy crops and damage trees 1.(be) a serious problem in many parts of Africa. But there could be a simple and 2.(relative) safe way to stop them in their tracks, by 3.(employ) the talents of the tiny African honeybees.

The number of elephants in Kenya 4.(rise) over the past ten years, with some disastrous results. Not only do these huge animals attack local farms 5. they step someone to death an average of once every two weeks. Now a biologist has discovered a possible threat: the elephants are extremely aware 6. trees that house honeybee hives.

''I am 7.(puzzle) why nobody has thought of this before, ''says Fritz Vollrath at the University of Oxford. ''When an elephant knocks a beehive he runs and the bees will follow him for miles. '' He says that placing hives of the honeybees around local farms could be 8. effective way to threaten the animals. Vollrath put 30 unoccupied and 6 occupied 9.(beehive) in some of the trees growing on a bush frequently damaged by elephants. On average the elephants attacked 9 out of 10 trees that didn’t have hives. But 10.(them) avoided all six trees with occupied hives. The elephants also left along one–third of the trees with empty hives, suggesting that some had learnt not to take the risk.