18. Boat 1 goes directly across a stream a distance L and back taking a time t₁. Boat 2 goes down stream a distance L and back taking a time t₂. If both boats had the same speed relative to the water, which of the following statements is true? (A) t₂ ＞ t₁ (B) t₂ ＜ t₁ (C) t₁ = t₂ (D) t₂ = 2t₁ (E) t₂ = 0.5 t₁