Input an expression for the ambulance's speed, v_s , in terms of the frequencies and the speed of sound v .

a) v_s = f₁ + f₂2
b) v_s = √f₁ ⋅ f₂
c) v_s = v ⋅ (f₁ + f₂)
d) v_s = 2vf₁ + f₂