The pair of compounds,which cannot exist together in a solution is

$A$ wire elongates by $l \, mm$ when a load $W$ is hung from it. If the wire goes over a pulley and two weights $W$ each are hung at the two ends,the elongation of the wire will be (in $mm$)

Which one of the following frequencies of radiation (in $Hz$) has a wavelength of $600 \ nm$?

$A$ $4 \mu F$ capacitor is charged by a $200 \ V$ battery. It is then disconnected from the supply and connected to another uncharged $2 \mu F$ capacitor. During the process,the loss of energy (in $J$) is:

Let $n = 1! + 4! + 7! + \ldots + 400!$. Then the ten's digit of $n$ is

Assertion : Both $Be$ and $Al$ can form complexes such as $BeF_4^{2-}$ and $AlF_6^{3-}$ respectively,$BeF_6^{3-}$ is not formed.
Reason : In case of $Be$,no vacant $d-$orbitals are present in its outermost shell.