船A重P_A，以速度 <img src="http://image.zhihuishu.com/zhs/onlineexam/ueditor/201812/1e6a475774084108a4d9ded2c70e6ca1.png" />航行。重P_B的物体B以相对于船的速度<img src="http://image.zhihuishu.com/zhs/onlineexam/ueditor/201812/b39c5c7163f846e8bc34287ec16ef1a7.png" />空投到船上，<img src="http://image.zhihuishu.com/zhs/onlineexam/ueditor/201812/20f0fb6f6dbc40638b69c7a33e96fd41.png" />设与水平面成<img src="http://image.zhihuishu.com/zhs/onlineexam/ueditor/201812/3651093fa407458596d5b85ce9b1dc5d.png" />角，且与<img src="http://image.zhihuishu.com/zhs/onlineexam/ueditor/201812/14fe1be64d75470c8843680bac3209a7.png" />在同一铅直平面内。若不计水的阻力，则二者共同的水平速度为____________。<img src="http://image.zhihuishu.com/zhs/onlineexam/ueditor/201812/9456cc95f694413584344f4bad82d008.png" /> A: <img src="http://image.zhihuishu.com/zhs/onlineexam/ueditor/201901/910e6572478f418c97aa55ae3e71eb87.png" /> B: <img src="http://image.zhihuishu.com/zhs/onlineexam/ueditor/201901/eb921b41a733425c811ac7b3848f3afe.png" /> C: <img src="http://image.zhihuishu.com/zhs/onlineexam/ueditor/201901/06d430260f9c402eaf641a0ba96427c5.png" /> D: <img src="http://image.zhihuishu.com/zhs/onlineexam/ueditor/201901/4c6f547166aa4497b9c6e8ff20fb5407.png" />