The equilibrium constant for the reaction $N_{2(g)} + O_{2(g)} \rightleftharpoons 2 NO_{(g)}$ at temperature $T$ is $4 \times 10^{-4}$. The value of $K_C$ for the reaction $NO_{(g)} \rightleftharpoons \frac{1}{2} N_{2(g)} + \frac{1}{2} O_{2(g)}$ at the same temperature is:

$A$ ball rests upon a flat piece of paper on a table top. The paper is pulled horizontally but quickly towards the right as shown. Relative to its initial position with respect to the table,the ball :-
$(A)$ Remains stationary if there is no friction between the paper and the ball.
$(B)$ Moves to the left and starts rolling backwards,i.e.,to the left if there is friction between the paper and the ball.
$(C)$ Moves forward,i.e.,in the direction in which the paper is pulled if there is friction between the paper and the ball.
Here,the correct statement$(s)$ is/are :-

In a triangle,if $b=20, c=21$ and $\sin A=\frac{3}{5}$,then $a$ is equal to :

Which of the following statements is not true?

The bond dissociation energy of $B-F$ in $BF_3$ is $646 \ kJ \ mol^{-1}$ whereas that of $C-F$ in $CF_4$ is $515 \ kJ \ mol^{-1}$. The correct reason for higher $B-F$ bond dissociation energy as compared to that of $C-F$ is