Two equal and opposite forces each $F$ act on a rod of uniform cross-sectional area $a$ as shown in the figure. The shearing stress on the section $AB$ will be

• A
  $\frac{F \sin \theta \cos \theta}{a}$
• B
  $\frac{F \sin \theta}{a}$
• C
  $\frac{F \cos \theta}{a}$
• D
  $\frac{F \sin ^2 \theta}{a}$