The Doctor had trained the snake, probably with the milk, to return at the sound of a whistle, the very whistle which you and your late sister heard.
医生训练好了蛇 可能用的就是我们见过的牛奶 让蛇在听到口哨声后就爬回去 也就是你和你死去的姐姐听到的那种声音