这条鱼用它的鳍在水中快速游动。

The fish swims quickly through the water using its ___fin.

潜水艇的鳍状结构帮助它在水下保持平衡。

The submarine's ___fins help it stay balanced underwater.

我给了他一张五美元钞票买汽水。

I gave him a ___fiver to buy some soda.