What does the following figure represent?

$EMF$ of the following concentration cell will be $............$ $V$
$Cl_2(1 \ atm) | Cl^{-}(1 \ M) || Cl^{-}(0.1 \ M) | Cl_2(2 \ atm)$

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 $NaOH$

$A$ class has $15$ boys and $5$ girls. Suppose $3$ students are selected at random from the class. The probability that there are $2$ boys and $1$ girl is

How is the rate of an enzymatic reaction increased?

Select the correct pair from the following.