It was difficult to（）how many trees had been destroyed. ["occupy","test","estimate","evaluate"]
形成酸雨的主要空气污染物是什么？
使用线条工具绘制直线线段时，按住（）可以绘制出水平线段。 ["Alt","Ctrl","Shift","Alt+Shift"]
人类排入大气中严重破坏臭氧层的是什么？
整车在平直线路检查中，有棚顶铁路货车车体倾斜不大于（）。 ["30mm","35mm","40mm","50mm"]
最小流量再循环的作用是什么？
