a water softener的用法和样例：
- One way to counteract hard water is by using a water softener.
- It has two skylites, tank less water heater, and a water softener system.
- Calcium citrate is also used as a water softener because the citrate ions can chelate unwanted metal ions.
- A water power station must have a dam.
- Every apartment house has a water tank on its roof.