$A$ pair of perpendicular straight lines passes through the origin and also through the point of intersection of the curve $x^2+y^2=4$ with $x+y=a$. The set containing the value of $a$ is

Which of the following pairs acts as a biofertilizer?

$2 \tanh^{-1} \frac{1}{2}$ is equal to

Half-life of a radioactive substance is $20 \, min$. The difference between the time when it is $33 \%$ disintegrated and $67 \%$ disintegrated is approximately ......... $min$.

The compound which does not show paramagnetism is :-