已知x=2是方程x²﹣a²=0的一个根，则a的值是（  ）

中国是礼仪之邦，礼仪文化历史悠久，博大精深。为弘扬中国的礼仪文化，学校准备举行“礼文化”为主题的活动，请你参加。

一宇宙飞船以v=1.0×10⁴ m/s的速度进入密度为ρ=2.0×10^﹣7 kg/m³的微陨石流中，如果飞船在垂直于运动方向的最大截面积为S=5m²，且认为微陨石与飞船碰撞后都附着在飞船上．为使飞船的速度保持不变，飞船的牵引力应为（   ）

为得到函数$y=\cos (2x+\frac{\mathrm{\pi }}{3})$的图像，只需将函数y=sin2x的图像(     )

读短文并按要求完成相应任务
    Today was the first day of the summer holiday．My friends and I went out together．We went swimming in the swimming pool．We swam for an hour．I did not swim well，but I enjoyed it very much．We had lunch in a fast—food restaurant．I had a sandwich and some orange juice．Alice had a hamburger．Jack had a lot of French fries and some cola．We went shopping in a supermarket．I bought a pink dress．Alice bought a pretty hat．Jack did not buy anything．
    We went home at 3 o'clock in the afternoon．We 、were tired，but we were happy．