A school bus is a bus used to take children to school . The school bus is usually the major means （工具）of transportation        children .
In North America the school bus is different from   buses  . The United States has specially built , painted and equipped（装备）school buses . They are usually painted yellow .
In 1939 , people in the USA    to paint the school buses yellow . In April of that  , Dr Frank , a professor at Columbia University , organized a   .  He and the other experts （专家）made national school bus standards at the meeting . The color was chosen     black lettering on that hue （颜色）was the easiest to
  in the early morning and late afternoon .
The meeting lasted for seven       . They talked about the      and the color of the school bus . They made a total of 45 pieces of standards at the meeting . And most of the standards are      used