Sigtran不是一个协议，而是一个（）。
参考答案：协议栈
读一读，填空。
One duck has a mouth, two eyes and two legs.
Two ducks have two mouths, four eyes and four legs.
1. Three ducks have mouths, eyes and legs.
2. Four ducks have mouths, eyes and legs.
(one two three four five six seven eight)
