[单选题]
When a beam of light (wavelength = 590piz4/+edudv )bl5 a 6vgw vxzm)h1k r:4c4i v0nm), originally traveling in air, enters a piece of gvm1vrizh cg w4x:)k0 4lass (index of refraction 1.50), its frequency
A. increases by a factor of 1.50.
B. is reduced to 2/3 its original value.
C. is unaffected.
D. none of the given answers