The work function of the photosensitive material is $4.0\text{eV}$. The longest wavelength of light that can cause photoelectric emission from the substance is (approximately):

1. A
   $3100\text{nm}$
2. B
   $966\text{nm}$
3. C
   $31\text{nm}$
4. D
   $310\text{nm}$