根据以下资料，回答15~18题。

The California condor is the largest bird in North America．It is an endangered species．20 years ago there were less than 30 birds left．All of the birds were trapped and kept in safe places．The number of condors grew larger．Puppets that looked like condors were used to take care of babies so that more eggs could be hatched.

In the last few years, some of the condors have been returned to the wild．But they seem to like humans now and come into danger around people．Some birds perch on houses and tear up roofs．Some have even gone into houses．Birds have been shot, poisoned, and killed by power lines．

Now there is another problem．Condors find dead animals that hunters have killed．The bullets cause lead to get into the meat and it can kill the condors．Some people think hunters should use different bullets．Other people think that the birds should not be put into the wild.

Why is lead poisoning a new problem for condors?

你的朋友(Tom)请你(Li Yuan)周末和他一起去承德的家里玩儿，但你有事不能去。请你用英文给他写一封信，内容包括：

(1)感谢他的邀请。

(2)说明不能去的原因。

(3)为不能去表示遗憾。

请在第_____处填上正确答案。

请在第_____处填上正确答案。

根据以下资料，回答1~5题。

请在第_____处填上正确答案。

根据以下资料，回答19~22题。

It is possible that the weather of the world may be changing．Some scientists imagine that this could even mean the beginning of another ice age．The effects of such a change in weather on human population of the world would be frightening.

Recent research suggests that the warming trend (倾向 ) of the past hundred years or so may be coming to an end．During the past ten years, scientists tell us that the temperature of the world has dropped a little．This would have effects on wind and rain in the weather picture.

Scientists believe clouds may be an important factor (因素) in changing the amount of heat on the earth．Another possibility is that man's agriculture and industry ways may affect the natural weather.

The weather of the world seems to be getting __.

The temperature of the world in the past ten years has dropped a little __.

The main factor in changing the amount of heat on the earth is __.

If the condor population has tripled since two decades ago, about how many are there now?

The warming weather of the past hundred years may __.

根据以下资料，回答15~18题。

The California condor is the largest bird in North America．It is an endangered species．20 years ago there were less than 30 birds left．All of the birds were trapped and kept in safe places．The number of condors grew larger．Puppets that looked like condors were used to take care of babies so that more eggs could be hatched.

In the last few years, some of the condors have been returned to the wild．But they seem to like humans now and come into danger around people．Some birds perch on houses and tear up roofs．Some have even gone into houses．Birds have been shot, poisoned, and killed by power lines．

Now there is another problem．Condors find dead animals that hunters have killed．The bullets cause lead to get into the meat and it can kill the condors．Some people think hunters should use different bullets．Other people think that the birds should not be put into the wild.

Why is lead poisoning a new problem for condors?