17．Nestwatch is a program to monitor nests（观察鸟巢）．It was developed by the Smithsonian Migratory Bird Center．
（56）BEnvironmental problems are one of the things that cause the decline（减少） in the number of the kinds of birds．Nestwatch can help us to better understand the causes of the decline of the kinds of birds and give possible solutions to it．It aims to provide a nest-monitoring plan to follow the breeding patterns（繁殖模式） of all North American birds．
（57）DNestwatch teaches people about the breeding of birds and encourages them to collect information on nests．The information includes where nests are，the kinds of nests and the number of eggs in them．
（58）CAnyone can check information on Nestwatch freely．Nestwatch makes it easy for people both to deal with their information on nests and to see other people's information on nests．
（59）AWe invite people of all ages who are interested in the natural world．Anyone is welcome to help monitor the success of the breeding of birds and better understand the danger they face．
（60）FThe information from lots of people will be put together for people to learn more about the breeding of birds and the things that influence their populations across different places．
Choose the best topic from A to F according to the meaning of the passage．
