[单项选择]

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（　）
A. Hunters shoot condors.
B. Puppet rearing practices have caused it.
C. Hunter’s bullets contain lead which poisons the meat condors eat.
D. They never used to like lead.