[单选题]
Monochromatic light of wavelength λ is diffracted while passing through a narrow slit. The three labeled light rays emerge from the top, halfway and bottom of the slit meet at a first diffraction minimum on a distant screen.
A. $\frac{1}{4}λ$
B. $\frac{1}{2}λ$
C. λ
D. $\frac{3}{2}λ$
