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.

• A
  Both statements $A$ and $B$ are wrong.
• B
  Statement $A$ is correct while statement $B$ is wrong.
• C
  Statement $B$ is correct while statement $A$ is wrong.
• D
  Both statements $A$ and $B$ are correct.