Choice B is the correct answer. Opposite directions → add speeds.

Let $t$ = time in hours.

Combined rate: $3 + 4 = 7$ mph

Equation: $7t = 10.5$$t = 1.5$

Solution: 1.5 hours

Verification: In 1.5 hours: $3(1.5) + 4(1.5) = 4.5 + 6 = 10.5$ miles ✓

💡 Strategic Tip: When moving apart, add speeds to find separation rate.

**Other choices fail verification.