Which of the following has the lowest boiling point?

Modern farmers have been able to increase rice production by $50\%$ using which of the following?

$A$ photon of energy $8\, eV$ is incident on a metal surface of threshold frequency $1.6 \times 10^{15}\, Hz$. The maximum kinetic energy of the emitted photoelectrons is ............. $eV$ $(h = 6.6 \times 10^{-34}\, Js)$.

The relative reactivities of acyl compounds towards nucleophilic substitution are in the order of

The work done by a force $\vec F = (-6x^3 \hat i) \text{ N}$,in displacing a particle from $x = 4 \text{ m}$ to $x = -2 \text{ m}$ is .............. $J$.

If the direction cosines of two lines are given by $l+m+n=0$ and $l^2-5m^2+n^2=0$,then the angle between them is