Which of the following is the path difference for destructive interference?

Two waves of same intensity $I_0$ emitted from two sources having same phase difference $(\phi)$. Due to superposition of two waves, the intensity of resultant wave is directly proportional to . . . . . . .

Interference cannot be observed in two sodium lamps of equal power because the two waves do not have a .....

Two monochromatic light beams of intensity $16$ and $9$ units are interfering. The ratio of intensities of bright and dark parts of the resultant pattern is

Two light beams of intensities $I$ and $4I$ produce an interference pattern on a screen. If the phase difference between them at point $A$ is $\pi/2$ and at point $B$ is $2\pi$,find the difference between the resultant intensities at points $A$ and $B$.

In the figure below,$P$ and $Q$ are two equally intense coherent sources emitting radiation of wavelength $20\, m$. The separation between $P$ and $Q$ is $5\, m$ and the phase of $P$ is ahead of that of $Q$ by $90^{\circ}$. $A, B$ and $C$ are three distinct points of observation,each equidistant from the midpoint of $PQ$. The intensities of radiation at $A, B, C$ will be in the ratio