钢轨焊后正火处理，从根本上是改变了焊缝的（），以改善钢轨焊缝性能。
A.内部组织结构
B.晶粒大小
C.化学成分
D.内部缺陷
参考答案：A
— This is the first time I ______ my first picture with my own hands.
— It's time that you ______ a picture for me.[ ]
A. took; took
B. have taken; took
C. took; will take
D. will take; have taken
造成长江中上游地区水土流失加剧的根本原因是（）
A.气候变迁，森林减少
B.流域内土质疏松
C.滥伐森林，陡坡开荒
D.降水丰沛，多暴雨
