下图为环境、资源和人口关系示意图，有关图中甲、乙、丙、丁含义的叙述，正确的是（    ）

测得生活中一些液体的pH如下表：

新浪微博上一个自称“住大别墅，开玛莎拉蒂”的20岁女孩郭美美高调炫富，招到几乎全体网民的质疑和批评。这启示我们（   ）

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．

如图所示,在光滑水平桌面上有一链条,共有(P+Q)个环,每一个环的质量均为m,链条右端受到一水平拉力F.则从右向左数,第P个环对第(P+1)个环的拉力是(   )