题目

The reason why the dinosaurs died out remains unknown. They ______ by an enormous volcanic eruption.A. would have killed                    B. might have been killedC. should have been killed            D. must have killed 答案：B下列事例中，属于增大压强的是（ ）A．拖拉机耕田时要安上两条履带B．铁路工人将铁轨铺在枕木上C．滑雪运动员要穿上滑雪板D．妈妈在切菜之前将刀刃磨锋利