题目

       In nineteen ninety-nine, twelve percent of public elementary schools in the United States required students to wear uniforms. Just three years later, estimates were almost double that.        A study of six big-city Ohio public schools showed students who were required to wear uniforms had improved graduation, behavior and attendance rates. Academic performance was unchanged.        Some middle and high schools in Texas have also joined the movement. Yet studies find mixed results from requiring uniforms. And some schools have turned away from such policies.      秒针在钟面上从“1”走到“6”，用了（　　）秒．A．10秒B．20秒C．25秒