你是一位物理老师，需要给学生进校这个题目的讲解，麻烦给出一个讲解视频，逐一对每个选项进行分析，给出正确的答案。（答案：C、D。分析D选项时，视频中给出速度合成的动态图）---**Question 1 (Multiple Choice)** **Question Stem:** 轮渡是过去人们过河经常采用的一种交通方式。如图是摆渡船轮渡的情景；已知船在静水中的速度为 v₁=10m/s，当船在 A 处，船头与上游河岸夹角 θ = 53° 过河，经过一段时间正好到达正对岸 B 处。已知河宽 d = 200m，船在静水中速度大小不变，水流的速度不变，则下列说法中正确的是 ( ) **Translation of Question Stem:** Ferry is a mode of transportation often used by people in the past to cross rivers. The figure shows the scenario of a ferry boat crossing a river. It is known that the boat's speed in still water is v₁=10m/s. When the boat is at point A, the boat's heading forms an angle θ = 53° with the upstream river bank to cross the river, and after some time it reaches point B directly opposite on the other bank. It is known that the river width is d = 200m. The boat's speed in still water remains unchanged, and the water flow velocity remains unchanged. Which of the following statements is correct? ( ) **Given Information:** Boat speed in still water: v₁ = 10 m/s Angle between boat's heading (v₁) and the upstream river bank: θ = 53° Starting point: A Ending point: B (directly opposite A) River width: d = 200 m **Options:** A. 此时船的渡河时间为 20s B. 此时河中水流速度为 8m/s C. 若船选择以最短时间过河，船最短的渡河时间为 20s D. 若水流速度突然增大变为 15m/s，则小船重新调整方向也无法到达正对岸 B 处 **Translation of Options:** A. The time taken for the boat to cross the river at this moment is 20s. B. The water flow velocity in the river at this moment is 8m/s. C. If the boat chooses to cross the river in the shortest time, the shortest crossing time is 20s. D. If the water flow velocity suddenly increases to 15m/s, the boat cannot reach point B directly opposite even by adjusting its direction. **Image Description:** * **Type:** Diagram illustrating a river crossing scenario. * **Main Elements:** * Two parallel horizontal lines representing the river banks. The upper bank is labeled with point B, and the lower bank is labeled with point A. * A dashed vertical line connecting point A on the lower bank to point B on the upper bank, indicating that B is directly opposite A. This line represents the river width. * Text label "上游" (Upstream) is on the left side of the diagram, pointing upstream. * Text label "水流" (Water flow) is on the right side, with three horizontal arrows pointing to the right, indicating the direction of the water current. * A vector labeled v₁ originates from point A. It points upwards and slightly to the left (towards upstream) relative to the horizontal river banks. This vector represents the boat's velocity in still water. * An angle θ is shown between the vector v₁ and the lower river bank line (which is parallel to the upstream direction). * **Labels and Annotations:** A, B, v₁, θ, 上游, 水流. * **Relative Position and Direction:** Point B is directly above point A. The water flows horizontally from left to right. The boat's velocity relative to water (v₁) is directed upwards and to the left from point A. The angle θ is between the vector v₁ and the upstream direction (parallel to the banks). **Mathematical Formulas/Equations (Implied from problem description):** Vector addition of velocities: resultant velocity (boat relative to ground) = boat velocity relative to water + water velocity relative to ground. Crossing time = river width / velocity component perpendicular to the banks. Drift = water velocity * crossing time. Note: The angle θ = 53° is given. It's usually assumed that sin(53°) ≈ 0.8 and cos(53°) ≈ 0.6.