What is $X$ in the following reaction?
$MgCl_2 + 2H_2O \to X + 2HCl + H_2$

The length of the subtangent at any point of the curve $x^m y^n = a^{m+n}$ is proportional to -

Choose the correct statement$(s)$:
$(A)$ The position of the centre of mass of a system is dependent on the choice of the coordinate system.
$(B)$ Newton's second law of motion is applicable to the centre of mass of the system.
$(C)$ Internal forces cannot change the state of the centre of mass.
$(D)$ Internal forces can change the state of the centre of mass.

If $A$ is the areal velocity of a planet of mass $M$,its angular momentum is

Which of the following elements does not show allotropy?

In a transistor,if $\frac{I_C}{I_E} = \alpha$ and $\frac{I_C}{I_B} = \beta$. If $\alpha$ varies between $\frac{20}{21}$ and $\frac{100}{101}$,then the value of $\beta$ lies between: