Which of the following is not correct?

$A$ particle is tied to a $20\, cm$ long string. It performs circular motion in a vertical plane. What is the angular velocity of the string when the tension in the string at the top is zero? (Take $g = 9.8\, m/s^2$)

$\left|\begin{array}{lll}24 & 25 & 26 \\ 25 & 26 & 27 \\ 26 & 27 & 27\end{array}\right|$ is equal to

In $YDSE$,the separation between the slits is halved and the distance between the slit and the screen is doubled. The fringe width is

The magnetic susceptibility of the material of a rod is $499$. Permeability of vacuum is $4 \pi \times 10^{-7} \ H/m$. Absolute permeability of the material of the rod in $H/m$ is:

The work functions of metals $A$ and $B$ are in the ratio $1 : 2$. If light of frequencies $f$ and $2f$ are incident on the surfaces $A$ and $B$ respectively,the ratio of the maximum kinetic energy of photoelectrons emitted is ($f$ and $2f$ are both frequencies greater than the threshold frequency of metals $A$ and $B$ respectively).