题目

第二节 (共5小题；每小题2分，满分10分)     根据短文内容，从短文后的选项中选出能填入空白处的最佳选项。选项中有两项为多余选项。 The organization，Keep Time for Children，was started in 2002 to make people realize the importance of family time on weekends，particularly for families with school age children．  51   We will try to help parents take one day off on weekends to be with their family．       52  What children want and need most is to spend time with their parents，not with toys or taking part in activities．But wit 运用声呐系统可以探测海洋深度。在与海平面垂直的方向上，声呐向海底发射         。（填“超声波”“次声波”“电磁波”）如果经4s接收到来自大海底的回波信号．则该处的海深为__________m（海水中声速是1500m/s）．但是，声呐却不能用于太空测距（比如地球与月球的距离）．这是因为________________________。