使用梯子前必须检查，确保安全可靠，如梯子有松动、破裂、磨损或腐朽现象禁止使用
参考答案：对
Schools across China are expected to hire 50,000 college graduates this year as short-term teachers, almost
three times the number hired last year, _____ reduce unemployment pressures. [ ]
A. help
B. to have helped
C. to help
D. having helped
A．a′=a，F1′=F1
B．a′＞a，F1′=F1
C．a′＜a，F1′=F1
D．a′＞a，F1′＞F1
