Which of the following oxides is different from the other three oxides?

$A$ $TV$ transmitter has a range of $50 \ km$. The height of the $TV$ transmitter is . . . . . . (Radius of the earth,$R_e = 6.4 \times 10^6 \ m$). (in $m$)

Mortar is a mixture of which of the following?

$A$ whistle of frequency $540 \ Hz$ rotates in a horizontal circle of radius $2 \ m$ at an angular speed of $15 \ rad/s$. What is the highest frequency heard by a listener at rest with respect to the centre of the circle (in $Hz$)? (Velocity of sound in air $= 330 \ m/s$)

If $x-y+1=0$ meets the circle $x^2+y^2+y-1=0$ at $A$ and $B$,then the equation of the circle with $AB$ as diameter is

In the circuit shown in the figure,the power dissipated as heat in the $6\,\Omega$ resistor is $6\,W$. What is the value of resistance $R$ in the circuit? $\Omega$