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

Considering interference between two sources of intensities $I$ and $4I$,the intensity at a point where the phase difference is $\pi$ is $(\cos \pi = -1)$.

Two coherent sources must have .......

In a double-slit interference experiment,if the path difference at a point on the screen for yellow light is $3\lambda/2$,then the fringe at that point will be . . . . . .

Consider the following statements about interference of light.
$A$-When crest of one wave coincides with crest of another wave at a point,this point is a point of destructive interference.
$B$-Two coherent sources emit waves of the same frequency with a constant phase difference.
Choose the correct option from the following.

Two waves having same intensity $I_{0}$ and originated from two non-coherent sources superpose at a point. The average intensity at that point is . . . . . . .