他愁眉苦脸地盯着账单。

He stares at the ___bills with a frown.

那只鸭子用喙梳理自己的羽毛。

The duck preens its feathers with its ___bill.

钱包里只剩几张五元的钞票了。

There are only a few five-dollar ___bills left in the wallet.

国会正在讨论一项关于环境保护的新法案。

The congress is debating a new ___bill on environmental protection.