A、Restricting the number of visitors. B、Raising money for his expanding operation. C、Accumulating wealth for himself. D、Raising

Steve Nichols has a way with birds. The founder of the UK’s National Parrot Sanctuary connects with them in a way few people can understand. Strolling through the centre at Friskney, in Lincolnshire, he can individually name most of the 414 parrots under his care. The majority of birds at the center are "problem pets" which their owners gave away. Mr. Nichols said increasing numbers of owners were finding their parrots too much to handle and were giving them away to the sanctuary. At the current rate, he expects the sanctuary’s stock to go up to more than 1,000 birds within two years. It is noise and aggression that usually drive most people to give up their birds. When BBC News Online visited the sanctuary, one woman phoned Mr. Nichols about giving away her bird after owning it for just nine days—despite paying ￡900 for it. The bird had started attacking family members. Mr. Nichols says: "Most people just don’t realize what they are getting themselves into when they buy parrots... they are still wild animals and can be very difficult." Mr. Nichols changes his mobile phone ring tone every couple of days to stop his parrots imitating it. It’s not like dogs which have been living with humans for thousands of years. [18]In recent weeks, Mr. Nichols has opened his sanctuary to paying customers, mainly in the hope of raising money for his expanding operation, which also includes a 24-hour helpline for worried owners.
16 Where are the birds at the Sanctuary mostly from?
17 Why does Mr. Nichols change his mobile phone ring tone very often?
18 What does Mr. Nichols charge visitors for?