Two point sources $S_1$ and $S_2$ separated by a distance $10 \mu m$ emit light waves of wavelength $4 \mu m$ in phase. $A$ circular wire of radius $40 \mu m$ is placed around the sources as shown in the figure,where $O$ is the centre of the circle and $OS_1 = OS_2$. Determine the nature of the interference at points $A, B, C,$ and $D$.

• A
  Points $A$ and $B$ are dark and points $C$ and $D$ are bright
• B
  Points $A$ and $B$ are bright and points $C$ and $D$ are dark
• C
  Points $A$ and $C$ are dark and points $B$ and $D$ are bright
• D
  Points $A$ and $C$ are bright and points $B$ and $D$ are dark