Answer the following question given this paragraph: 

Perhaps the most familiar liquid-solid solution is dental amalgam, used to fill teeth when there is a cavity. Approximately 50% of the amalgam material is liquid mercury to which a powdered alloy of silver, tin and copper is added. Mercury is used because it binds well with the solid metal alloy. However, the use of mercury-based dental amalgam has gone under question in recent years because of concerns regarding the toxicity of mercury.


Q: The use of mercury-based dental amalgam has gone under question in recent years because of concerns regarding what?

 Choices:

- the expense

- the oxygen

- the variability

- the toxicity

A:
the toxicity
