Which one of the following removes temporary hardness of water?

$A$ wire of length $l$ is bent into a circular loop of radius $R$ and carries a current $I$. The magnetic field at the centre of the loop is $B$. The same wire is now bent into a double loop of equal radii. If both loops carry the same current $I$ and it is in the same direction,the magnetic field at the centre of the double loop will be

The two curves $x=y^2$ and $xy=a^3$ cut orthogonally at a point, then $a^2$ is equal to

The product of the lengths of the perpendiculars from any point on the hyperbola $x^2-y^2=8$ to its asymptotes is

If a wire of length $L$ forms a loop of radius $R$ and has $n$ turns,find the magnetic field at the center of the loop. The current flowing in the loop is $I$.

How many $sp^2$ and $sp$-hybridised carbon atoms are present respectively in the following compound?