What will be the $pH$ of an aqueous solution of $MgCl_2$?

Consider the following statements:
$(a)$ $R_2SiO$ is the repeating unit of linear silicones.
$(b)$ $RSiCl_3$ on hydrolysis followed by dehydration gives linear silicones.
$(c)$ Silicones can be used as heat insulators.
$(d)$ Silica is soluble in $HF$.
The correct statement$(s)$ is/are:

Which of the following is a self-adjusting force?

The number of subsets of $\{1, 2, 3, \ldots, 9\}$ containing at least one odd number is

Which of the following substances is obtained from the fungus $Claviceps$ $\text{purpurea}$ (ergot)?

If $u=f(r)$,where $r^2=x^2+y^2$,then $\left(\frac{\partial^2 u}{\partial x^2}+\frac{\partial^2 u}{\partial y^2}\right)$ is equal to