Which of the following is $NOT$ a characteristic feature of the class $Mammalia$?

$A$ solid element $Y$ conducts electricity and forms two types of chlorides: $YCl_n$ (a colorless volatile liquid) and $YCl_{n-2}$ (a colorless solid). To which group does element $Y$ belong?

Consider the following statements regarding the major pigments and stored food in the different groups of algae and select the correct option.
$I.$ In Chlorophyceae, the stored food material is starch and the major pigments are chlorophyll-$a$ and chlorophyll-$b$.
$II.$ In Phaeophyceae, laminarin is the stored food and major pigments are chlorophyll-$a$ and chlorophyll-$c$.
$III.$ In Rhodophyceae, floridean starch is the stored food and major pigments are chlorophyll-$a$, $d$ and phycoerythrin.

$A$ thin wire of length $l$ and mass $M$ is bent in the form of a semicircle. What is its moment of inertia about an axis passing through the ends of the wire?

$A$ family of curves has the differential equation $x y \frac{d y}{d x}=2 y^2-x^2$. Then,the family of curves is

The eccentricity of the ellipse $\frac{x^2}{16} + \frac{y^2}{9} = 1$ is