讲解这道题---**Extraction Content:** **Question Stem:** 4. 小明乘坐在一做匀速直线运动的有轨电车上，他带了一个充入氦气的气球并把它固定在电车地板上.由于突发情况, 电车紧急刹车，结果在车厢内出现的情景是 （ ） **Options:** A. [Image A] B. [Image B] C. [Image C] D. [Image D] **Chart/Diagram Description:** The image displays a physics problem involving a boy, a helium balloon, and a train. The problem description is followed by four diagrams (labeled A, B, C, and D), each depicting the interior of a train car with a boy and a balloon inside. The train is drawn in profile, showing its wheels and two doors. There are handles hanging from the ceiling. A black arrow is shown above each train in options A, B, and C, indicating the direction of motion before braking (although this arrow is absent in D, the relative positions and angles of the figures within the train suggest the direction of motion/braking force). The boy is shown standing, and the balloon is attached to the floor by a string. **Diagram A:** * **Type:** Line drawing illustrating a scene inside a train car. * **Main Elements:** Train car (profile view), two doors, wheels, three hanging handles, a boy standing, a balloon on a string attached to the floor, a black arrow above the train pointing to the right. * **Details:** The boy is standing upright, facing forward (towards the right). The balloon is positioned slightly in front of the boy (relative to the direction of motion), and the string is angled slightly forward. **Diagram B:** * **Type:** Line drawing illustrating a scene inside a train car. * **Main Elements:** Train car (profile view), two doors, wheels, three hanging handles, a boy standing, a balloon on a string attached to the floor, a black arrow above the train pointing to the right. * **Details:** The boy is leaning forward (towards the right). The balloon is positioned behind the boy (relative to the direction of motion), and the string is angled backward. **Diagram C:** * **Type:** Line drawing illustrating a scene inside a train car. * **Main Elements:** Train car (profile view), two doors, wheels, three hanging handles, a boy falling backward, a balloon on a string attached to the floor, a black arrow above the train pointing to the right. * **Details:** The boy is shown in a falling posture, tilted backward (towards the left). The balloon is positioned behind the boy and is tilted backward relative to the vertical, with the string angled backward. **Diagram D:** * **Type:** Line drawing illustrating a scene inside a train car. * **Main Elements:** Train car (profile view), two doors, wheels, three hanging handles, a boy standing, a balloon on a string attached to the floor. (No explicit arrow indicating initial motion direction). * **Details:** The boy is standing upright, facing forward. The balloon is positioned in front of the boy, and the string is angled forward. (This diagram appears similar to diagram A in the positioning of the boy and balloon). *Note: The question describes the train making an emergency stop while moving at a uniform linear speed. The initial direction of motion indicated by the arrow in A, B, and C is to the right.*