It's a fine day. The Black（布莱克一家）are busy today. Tony and Gogo are going to the beach. Jenny is going to the park with her friends. Mrs. Black is going to the shopping mall. She is going to buy some milk and vegetables. Is Mr. Black going to the shopping mall, too? No, he isn't. He's going to stay at home. Look! He is reading the newspapers（报纸）.

在交通事故的分析中，刹车线的长度是很重要的依据。刹车线是汽车刹车后，停止转动的轮胎在地面上滑动时留下的痕迹。在某次交通事故中，汽车的刹车线长度是14m。假设汽车轮胎与地面间的动摩擦因数为0.7，g=10m/s²。则汽车开始刹车时的速度为(        )

如果以“两极格局的确立与解体”为题撰写专著，贯穿全书的主线应该是，美苏两国（   ）