What is the role of limestone during the extraction of iron from haematite ore?

The two surfaces of a biconvex lens have the same radii of curvature. This lens is made of glass with a refractive index of $1.5$ and has a focal length of $10 ~cm$ in air. The lens is cut into two equal halves along a plane perpendicular to its principal axis to yield two plano-convex lenses. The two pieces are glued such that the convex surfaces touch each other. If this combination lens is immersed in water (refractive index $=$ $4/3$),its focal length (in $cm$) is:

The number of unpaired electrons in $Zn^{2+}$ is

Given below is the $ECG$ of a normal human. Which one of its components is correctly interpreted below?

$A$ body projected vertically upwards crosses a point twice in its journey at a height $h$ just after $t_1$ and $t_2$ seconds. Maximum height reached by the body is

$A$ tangent galvanometer has a coil of $25$ turns and a radius of $15 \, cm$. The horizontal component of the earth's magnetic field is $3 \times 10^{-5} \, T$. The current required to produce a deflection of $45^o$ in it is.....$A$