Charge flow equals electron density times volume times charge per electron Steps: - Total charge Q equals number of free electrons times elementary charge e, where number of electrons is density n times volume A × d, with A the cross-sectional area and d the average displacement. - Rearrange formula to d = Q / (n e A). - Compute A = π (diameter / 2)^2 = π (2.1 × 10^{-6})^2 ≈ 1.39 × 10^{-11} m². - Substitute Q = 0.15 C, n = 8.5 × 10^{28} m^{-3}, e = 1.6 × 10^{-19} C to get d = 8.0 × 10^{-6} m. Why C is correct: - The formula d = Q / (n e A) from drift current definition directly yields 8.0 × 10^{-6} m when values are plugged in. Why the others are wrong: - A likely from using diameter instead of radius in area formula, underestimating A by factor of 4. - B from approximating π ≈ 3 and rounding n e incorrectly, leading to slight underestimation. - D from neglecting π in area calculation, overestimating A …