which situations might cause two observers (A and B) to measure different frequencies for the same vibrating object? select the two correct answers

Question

a. observer A and observer B are both stationary and at the same distance from the object
b. observer A is stationary and observer B is moving
c. observer A and observer B are stationary but at different distances from the vibrating object
d. observer A and observer B are moving at different speeds relative to each other

Answer 1

b. observer A is stationary and observer B is moving
d. observer A and observer B are moving at different speeds relative to each other