增强驾驶员的安全驾驶意识，自觉遵守交通规则是预防交通事故的（）。
A.首要任务
B.必要任务
C.条件之一
D.辅助手段
参考答案：A
There _____ great changes in our hometown in the past fifteen years. [ ]
A. has been
B. have been
C. was
D. were
制备高聚物/粘土纳米复合材料方法有（）和（）两种。
