The Difference Between Lightwaves Approaching and Lightwaves Receding Is

Multiple Choice

The difference between lightwaves approaching and lightwaves receding is

A) when approaching,lightwaves have higher frequency; when receding their frequency is lower.
B) when approaching,lightwaves have lower frequency; when receding their frequency is higher.
C) the frequency of approaching lightwaves remain constant,while receding lightwaves are invisible.
D) There is no difference