Which one of the following is used in the preparation of styrene?

$A$ force $\overrightarrow F = 6\hat i + 2\hat j - 3\hat k$ acts on a particle,causing a displacement of $\overrightarrow s = 2\hat i - 3\hat j + x\hat k$. If the work done is zero,find the value of $x$.

Match the following columns regarding the geological time scale of evolution:

Column-$I$	Column-$II$
$A$. $500$ million years ago	$1$. Sea weeds and few plants
$B$. $350$ million years ago	$2$. Jawless fish
$C$. $320$ million years ago	$3$. Invertebrates

$A$ body is executing simple harmonic motion. At a displacement $x$ its potential energy is $E_1$ and at a displacement $y$ its potential energy is $E_2$. The potential energy $(E)$ at displacement $(x+y)$ is

$A$ body of mass $1\,kg$ crosses a point $O$ with a velocity $60\,ms^{-1}.$ $A$ force of $10\,N$ directed towards $O$ begins to act on it. It will again cross $O$ in ........ $\sec$.

If the angle $2 \theta$ is acute,then the acute angle between the pair of straight lines $x^2(\cos \theta - \sin \theta) + 2xy \cos \theta + y^2(\cos \theta + \sin \theta) = 0$ is: