Which of the following is a hormone-releasing $IUD$?

Explain why alkyl groups act as electron donors when attached to a $\pi$ system.

The angle between the lines represented by $y^2 \sin^2 \theta - xy \sin^2 \theta + x^2(\cos^2 \theta - 1) = 0$ is

Which of the following is the weakest base?

If the sum of the distances of a point $P$ from two perpendicular lines in a plane is $1$,then the locus of $P$ is a

$A$ block of mass $M_1$ is hung by a light spring of force constant $k$ from the top bar of a reverse $U$-frame of mass $M_2$ resting on the floor. The block is pulled down from its equilibrium position by a distance $x$ and then released. The minimum value of $x$ such that the reverse $U$-frame will leave the floor momentarily is: