美元是可数名词还是不可数名词?

The dollar finished the day slightly down.

当日结束时美元汇率略有下降。

What's the rate of exchange against the dollar?

与美元的兑换率是多少？

Here's a five dollar bill ─ please keep the change.

这是一张五元的钞票——零钱就不用找了。

If you want the best, you have to pay top dollar.

你想要最好的，就得花大钱。

The US dollar went through a difficult time, in company with the oil market.

美元与石油市场同时经历了艰难时期。