如图所示，在一粗糙水平面上有两个质量分别为m₁和m₂的木块1和2，中间用一原长为L，劲度系数为k的轻弹簧连接起来，木块与地面间的滑动摩擦因数均为μ．现用一水平力向右拉木块2，当两木块一起匀速运动时两木块之间的距离是（   ）

向下列装置中加入活酵母粉，适宜温度培养一段时间后有酒精产生的装置是（　　）

下列行为中，属于鸟类繁殖过程必不可少的行为是（   ）

①交配  ②筑巢  ③化蛹  ④育雏  ⑤结茧  ⑥孵卵  ⑦跳舞  ⑧产卵．

     Mrs Smith is Jim's mother. She loves her family very much. She often buys food, fruit and clothes for Jim and his father. Now many clothes are on sale at a good price.
     Mrs Smith comes to the store and she wants to buy some clothes. They have sweaters in all colors for￥15 each and sports shores for only ￥28.Mrs Smith likes the red sweater and  she buys one for herself(她自己).She buys a pair of sports shores for her son. The T-shirts in the store are just ￥18! She buys a white one for Mr Smith. And that's not all. The socks, in all colors, are ￥2 each. She buys five pairs.