题目

    most wolves rely on their groups for food ,shelter and companionship ,they have developed a system of signals to keep the groups together. A．Because    B．Although   C．Unless      D．Where 答案：A我们可以用两种方法来判断物体做功的快慢，如图－2－1所示，用挖掘机挖土与人力挖土做功的快慢不同，这里所用的判断方法是：做功时间相同，比较__________．另一种判断方法是________相同，比较________________．