Choice B is the correct answer. Let $k$ = kayak speed, $c$ = current speed.

System: $\begin{cases} (k + c) \cdot 1.5 = 18 \\ (k - c) \cdot 3 = 18 \end{cases}$

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

Step 2: Add: $2k = 18$$k = 9$

Step 3: Find c: $9 + c = 12$$c = 3$

Solution: Current is 3 mph

Verification: Down: $(9+3) \times 1.5 = 18$ ✓, Up: $(9-3) \times 3 = 18$ ✓

💡 Strategic Tip: Current helps or hinders depending on direction.