At two points $P$ and $Q$ on screen in Young's double slit experiment. Waves from slits $S_1$ and $S_2$ have a path difference of $0$ and $\frac{\lambda }{4}$ respectively. The ratio of intensities at $P$ and $Q$ will be

Consider the following statements in case of Young's double slit experiment.

I. A slit $S$ is necessary, if we use an ordinary extended source of light.

II. A slit $S$ is not needed, if we use an ordinary but well collimated beam of light.

III. A slit $S$ is not needed, if we use a specially coherent source of light.

Which of the above statements are correct?