Light of wavelength $\lambda$ is incident on a single slit of width $a$,and the distance between the slit and the screen is $D$. In the diffraction pattern,if the slit width is equal to the width of the central maximum,then $D=$

• A
  $\frac{a^2}{\lambda}$
• B
  $\frac{a}{\lambda}$
• C
  $\frac{a^2}{2 \lambda}$
• D
  $\frac{a}{2 \lambda}$