隔离法灭火就是将燃烧的物体与周围的物质隔离开来，使燃烧最终没有（）而终止的一种灭火方法。
参考答案：可燃物
买一支钢笔需要a元，买一本笔记本需要b元，那么买支m钢笔和n本笔记本需要（ ）元。
By serving others，a person focuses on someone other than himself or herself，______ can be very eyeopening and rewarding.
A．who
B．which
C．what
D．that
