DISCOVERY
Using tech to count birds
Have you ever wondered how we count the number of birds on our planet? The answer to this question is more high-tech than you would think.
- 对本期电子报订户、有效期内的VIP教师会员和本学期高一尊享卡用户，我们开放所有内容：
- 如果您拥有以上权限，请点这里登录；
- 订阅电子报，请点这里；
- 激活VIP教师卡，请点这里；
- 激活21英语尊享卡，请点这里。
