Which of the following ions will cause hardness in water?

How is the oxygen atom surrounded in ice?

When zeolite,which is a hydrated sodium aluminium silicate,is treated with hard water,the sodium ions are exchanged with which of the following?

The ratio of the viscosity (in centipoise) of $D_2O$ to that of $H_2O$ at $25^{\circ} C$ is

Water softening by Clark's process uses

Identify the statements which are not correct from the following:
a) In the structure of ice,each oxygen atom is surrounded by $4$ other hydrogen atoms via hydrogen bonding.
b) Temporary hardness of water is due to dissolved $Mg(HCO_3)_2$ and $Ca(HCO_3)_2$,not $NaHCO_3$.
c) In the reaction of acidified $KMnO_4$ and $H_2O_2$,$H_2O_2$ acts as a reducing agent.
d) $3 \ g \ L^{-1}$ $H_2O_2$ is not equal to $100$ volume $H_2O_2$ (since $100$ volume $H_2O_2$ is approximately $303.7 \ g \ L^{-1}$).