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                                                                                                       月夜忆舍弟
                                                                                                            杜甫
                                                                                            戍鼓断人行，边秋一雁声。
                                                                                            露从今夜白，月是故乡明。
                                                                                            有弟皆分散，无家问死生。
                                                                                            寄书长不达，况乃未休兵。

物体静止在水平桌面上，则（   ）

如果一对夫妇第一胎生了一个女孩，那么第二胎生一个男孩的几率是（　　）