单项选择题
A. Asia B. the United States C. Europe D. both B and C
A. a stepladder used to have four legs. B. it is dangerous to use tools C. a tripod is safer in a tool design D. workers are safer on production lines
A. leave the school if they fail to respect a woman B. attend the preparatory classes in the summer before enrolling C. do their homework to review what they have learned D. come to the school on Sundays