Why does mercury cell gives a constant voltage throughout its life ?

Why a mercury cell gives a constant voltage throughout its life ?

Assertion (A) : (H_(2)+O_(2)) fuel cell gives a constant voltages throughout its life. Reason (R): In this fuel cell, H_(2) reacts with overset(c-)(Oh)H ions, yet the over all [overset(c-)(O)H] does not change.

Statement-I: H_(2)+O_(2) fuel cell gives a constant voltage throughout its life. Because Statement-II: In this fuel cell, H_(2) reacts with OH^(-) ions yet the overall concentration of OH^(-) ions does not change.

Which of the following cells has a constant voltage through-out its life?

Why does the cell potential of mercury cell remain constant throughout its life ?