Which amino acids are present in histones?

$A$ vessel contains oil (density $= 0.8 \, g/cm^3$) over mercury (density $= 13.6 \, g/cm^3$). $A$ homogeneous sphere floats with half of its volume immersed in mercury and the other half in oil. The density of the material of the sphere in $g/cm^3$ is

The structure which prevents the entry of food into the respiratory tract is

Out of $30$ points in a plane,$8$ of them are collinear. The number of straight lines that can be formed by joining these points is:

What are $X$ and $Y$ in the following reaction?
$CF_2Cl_2 \xrightarrow{UV} X + Y$

The locus of the centre of a circle which always passes through the fixed points $(a, 0)$ and $(-a, 0)$ is