$NaOH$ is a strong base because

Which of the following pairs of species is used in the preparation of antacid medicines?

Which of the following acts as a base according to the $Brønsted-Lowry$ theory?

The pair of compounds which cannot exist together in aqueous solution is:
$I.$ $NaH_2PO_4$ and $NaHCO_3$
$II.$ $Na_2CO_3$ and $NaHCO_3$
$III.$ $NaOH$ and $NaH_2PO_2$
$IV.$ $NaHCO_3$ and $NaOH$

Among $CO_3^{2-}$,$OH^{-}$,$NH_3$ and $HCO_3^{-}$,the species that acts as a Bronsted acid as well as a Bronsted base,is

The conjugate acid of $NH_3$ is