In the Bohr series of lines of the hydrogen spectrum,the third line from the red end corresponds to which one of the following inter-orbit transitions in an atom of hydrogen?

The cathode of a photoelectric cell is changed such that the work function changes from $W_1$ to $W_2$ $(W_2 > W_1)$. If the current before and after the change are $I_1$ and $I_2$ respectively,with all other conditions remaining unchanged,then (assuming $h\nu > W_2$):

Let $Q$ be the set of all rational numbers in $[0,1]$ and $f:[0,1] \rightarrow [0,1]$ be defined by $f(x) = \begin{cases} x & \text{for } x \in Q \\ 1-x & \text{for } x \notin Q \end{cases}$. Then,the set $S = \{x \in [0,1] : (f \circ f)(x) = x\}$ is equal to

$A$ pair of perpendicular straight lines passes through the origin and also through the point of intersection of the curve $x^2+y^2=4$ with $x+y=a$. The set containing the value of $a$ is

How many unit cells are present in $39 \ g$ of potassium that crystallises in body-centred cubic structure? [At. wt. of $K = 39$]

The following reaction occurs in the Blast Furnace where iron ore is reduced to iron metal:
$Fe_2O_{3(s)} + 3CO_{(g)} \rightleftharpoons 2Fe_{(l)} + 3CO_{2(g)}$
Using the Le Chatelier's principle,predict which one of the following will not disturb the equilibrium?