单项选择题
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
A.serious B.apparent C.severe D.fatal