某种物资在前后两批进厂的供应间隔期内，为保证生产正常进行所必需的、经济合理的物资储备数量。称为（）.
参考答案：经常储备定额
In the last few years, great changes _________ in China. [ ]
A. were happened
B. have been happened
C. have happened
D. happened
两个配合和谐的树种混交时，其种间各生长阶段变化少，没有矛盾。
