A diffraction grating contains $N$ lines per unit length and is set at $90^\circ$ to monochromatic light with wavelength $\lambda$. What expression gives $\theta$, the angle to the normal to the grating at which the third order diffraction peak is seen?
- A$\sin\theta = \frac{1}{3N\lambda}$
- B$\sin\theta = \frac{N\lambda}{3}$
- C$\sin\theta = 3N\lambda$
- D$\sin\theta = \frac{3\lambda}{N}$