当活塞上死点，缸内（）压力仍高于大气压力，排气门迟关一些，可使废气排得干净些。
A、空气
B、废气
C、可燃气体
D、二氧化碳
参考答案：B
海面有A，B，C三个灯塔，AB=10km，从A望C和B成60°的视角，从B望C和A成75°的视角，则BC=______km．
—I really don't know what I ________yesterday without the bike you lent me.
—Glad to have been some help to you.
A．could do
B．should have done
C．had done
D．could have done
