Choice B is the correct answer. Let $s$ = swimmer speed in still water and $c$ = current speed.

System: $\begin{cases} (s + c) \cdot 1 = 6 \\ (s - c) \cdot 2 = 6 \end{cases}$

Step 1: Simplify: $\begin{cases} s + c = 6 \\ s - c = 3 \end{cases}$

Step 2: Add equations: $2s = 9$$s = 4.5$

Step 3: Find $c$: $4.5 + c = 6$$c = 1.5$

Solution: Current is 1.5 mph

Verification: Down: $(4.5 + 1.5) \times 1 = 6$ ✓, Up: $(4.5 - 1.5) \times 2 = 6$ ✓

💡 Strategic Tip: Current aids downstream, opposes upstream.

**Other choices fail verification.