机械式封堵三通上下护板都需要安装（）
A、卡环机构
B、密封组件
C、塞柄机构
D、防转卡口
参考答案：B
水稻和陆稻属于土壤生态型。 ( )
Why are you still doing it? You should ______ it earlier. [ ]
A. have finished
B. not finish
C. finish
D. haven't finished
