Choice B is the correct answer. Let $c$ = canoe speed in still water and $r$ = river current speed.

System: $\begin{cases} (c + r) \cdot 2 = 24 \\ (c - r) \cdot 4 = 24 \end{cases}$

Step 1: Simplify: $\begin{cases} c + r = 12 \\ c - r = 6 \end{cases}$

Step 2: Add equations: $2c = 18$$c = 9$

Solution: Canoe speed is 9 mph in still water

Verification: Downstream: $(9+3) \times 2 = 24$ ✓, Upstream: $(9-3) \times 4 = 24$ ✓

💡 Strategic Tip: Current helps downstream, opposes upstream.

**Other choices fail verification.