In which of the following,ortho/para substitution by an electrophile is very facile?

$A$ motor is used to deliver water at a certain rate through a given horizontal pipe. To deliver $n$-times the water through the same pipe in the same time,the power of the motor must be increased as follows:

The relation between pressure $p$ and volume $V$ is given by $p V^{1/4} = \text{constant}$. If the percentage decrease in volume is $\frac{1}{2} \%$,then the percentage increase in pressure is:

The magnifying power of a refracting type of astronomical telescope is $m$. If the focal length of the eyepiece is doubled,the magnifying power will become:

The solution of the differential equation $\frac{dy}{dx} - y \tan x = e^x \sec x$ is

$A$ particle of mass $M$ is at a distance $a$ from the surface of a thin spherical shell of uniform mass $M$ and radius $a$. Which of the following statements is true regarding the gravitational field and potential inside the shell?