Doppler Effect for Approaching Sound Source Steps:

• Stationary case: Wavelength λ = v / f, where v is speed of sound in air, f is source frequency.
• Approaching source: Observed frequency increases to f' = f × (v / (v - v_s)), where v_s is source speed, so f' > f.
• Wave speed v remains constant relative to stationary air medium, independent of source motion.
• New wavelength λ' = v / f' < λ, so shorter wavelength.

Why C is correct:

• Speed of sound is fixed by the medium (air); formula v = f λ shows λ decreases with higher f to keep v constant.

Why the others are wrong:

• A: Wavelength shortens (not lengthens); speed unchanged (not greater).
• B: Speed unchanged (not greater).
• D: Wavelength shortens (not lengthens).

Final answer: C