An element reacts with hydrogen to form a compound $A$ which on treatment with water liberates hydrogen gas. The element can be

The coefficient of $x^k$ in the expansion of $\frac{1-2x-x^2}{e^{-x}}$ is

If $0 < p < q$,then $\lim _{n \rightarrow \infty}\left(q^n+p^n\right)^{1 / n}$ is equal to :

$A$ player tosses $2$ fair coins. He wins Rs. $5$ if $2$ heads appear,Rs. $2$ if $1$ head appears,and Rs. $1$ if no head appears. The variance of his winning amount is:

The dispersive powers of the materials of two lenses forming an achromatic combination are in the ratio of $4: 3$. If the effective focal length of the two lenses is $+60 ~cm$,then the focal lengths of the lenses should be:

Haversian canals are found in