①上皮组织  ②分生组织   ③结缔组织    ④肌肉组织   ⑤保护组织    ⑥神经组织

如图所示，在光滑水平面上有甲、乙两木块，质量分别为m₁和m₂，中间用一原长为L，劲度系数为k的轻质弹簧连接起来，现用一水平力F向左拉木块甲，当两木块一起匀加速运动时，两木块之间的距离是（   ）

阅读理解

   Tom is thirteen years old．He is in Grade Seven this year．He likes playing football and watching football matches．And he often reads newspapers．He does his best to know when and where there is going to be a football match．Now Tom is having

lunch and he is also reading newspapers．He is very happy because there is going to be a nice football match on TV at four this afternoon．He wants to watch it very much，but he is going to have English and maths lessons h the afternoon．He thinks hard and finds a way．

    “Hello，Mrs. Black，”Tom says to his teacher on the telephone．“Tom is ill in bed．H e wants to ask for half a day's leave(请半天假)．”

    “Oh，I'm sorry to hear that，”says Mrs. Black．“But who is that?”

    “It's my father，Mrs. Black，”Tom answers．

20世纪70年代末，我国思想理论界开展了关于真理标准问题的大讨论，使人们认识到实践是检验真理的唯一标准。这场大讨论为下列哪一次会议成功召开奠定了基础    （    ）