Total time = fall time + sound travel time

Steps:

• Calculate fall time: $t_f = \sqrt{\frac{2d}{g}} = \sqrt{\frac{2 \times 78}{9.8}} \approx 3.99$ s.
• Calculate sound time: $t_s = \frac{d}{v} = \frac{78}{340} \approx 0.23$ s.
• Add times: total $t = 3.99 + 0.23 = 4.22$ s.
• Match to choices: 4.22 s is option D.

Why D is correct:

• Uses free-fall equation $d = \frac{1}{2} g t^2$ with $g = 9.8$ m/s² and sound speed formula $t = \frac{d}{v}$.

Why the others are wrong:

• A: Ignores sound time, underestimates total.
• B: Fall time only with $g = 9.8$, misses sound.
• C: Uses $g = 10$ m/s², slightly overestimates fall time.

Final answer: D