Which of the following statements is correct?

Consider the following bromides:
The correct order of $S_N1$ reactivity is

The de-Broglie wavelength of a particle accelerated with $150 \ V$ potential is $10^{-10} \ m$. If it is accelerated by $600 \ V$ potential difference,its wavelength will be ........... $\mathring{A}$.

$A$ microscope consists of an objective of focal length $1.9 \ cm$ and an eyepiece of focal length $5 \ cm$. The two lenses are kept at a distance of $10.5 \ cm$. If the image is to be formed at the least distance of distinct vision $(25 \ cm)$,the distance at which the object is to be placed before the objective is: (in $cm$)

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

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: