为了探究浮力大小与液体密度的关系，老师准备了如图所示的器材供同学们选择，同学所选的四组器材中，能达到探究目的是（不能多选或少选器材）（　　）

    This is Kate Smith. She is 13 years old. She lives in Guangzhou. She studies in No.10 Middle school. Her parents work there. They are both English teachers. They are form the UK. She likes eating junk food. She doesn't like playing sports. She has one healthy habit: She sleeps for 9 hours every night.
    Kate Smith has a brother. His name is Jack Smith. He is 11 years old. He doesn't eat junk food. He likes playing sports. He often runs, plays basketball, and goes swimming. He eats fruit every day. He likes eating vegetables. But he has an unhealthy habit: He sleeps for 6 hours every night.

甲乙两地相距360千米，一轮船往返于甲、乙两地之间，顺水行船用18小时，逆水行船用24小时，若设船在静水中的速度为x千米/时，水流速度为y千米/时，则下列方程组中正确的是（　　）