下表列出了相同条件下不同物质密度及声音在其中传播的速度，根据表中提供的信息，可以得出的结论是（    ）

    I am looking at an old friend of mine.

    He has been one of my best____(friend )for almost 12 years. He has blessed my life with his loyalty, kindness____(wise) and love since the day when I first met him____doesn't matter to me that he has four legs instead of two, fur instead of clothes, and barks at me instead of talking to me.

    As I watch my friend sleeping____the sun, I cant help____(think) of all the goodness he has brought into my life. He has been with me every day through good and bad times. He has always been my protector. He has always reminded me that I am loved and showed me how____(love)others at the same time.

    My friend has aged far____(quickly)than me and had more and more health problems over the years. I am not sure how long we will be together before he ____(pass)away from this world.

    This ____(wonder)dog has helped me understand that you don't have to be perfect to be loved and____poor health and old age might affect the body but they don't have to decrease the spirit.

对细胞膜结构和组成的研究经历了一个漫长的过程．回答下列问题：

（1）早在19世纪末期，研究人员就曾选用500多种化学物质对细胞的透性进行了上万次的研究．发现：凡是溶于脂质的物质，很容易穿过膜；这表明：细胞膜是由____ 构成的．

（2）将红细胞加入蒸馏水中放置一段时间后，在该液体的____ （上、中、下）层能得到破碎的磷脂双分子层．1925年，有人用丙酮从红细胞膜中提取到磷脂，并将其铺在水面上形成磷脂单分子层．测量铺展面积，约为红细胞总表面积的两倍，说明细胞膜由____ 层磷脂分子构成．

（3）1933年发现：蛋白酶能够使细胞膜分解，这表明____．用红绿两种荧光蛋白分别标记人的细胞和小鼠的细胞，两种细胞融合成一个细胞后，于37℃中40分钟后两种颜色荧光蛋白均匀分布，说明细胞膜具有的结构特性：____ ．

（4）与植物细胞比较，构成动物细胞膜的重要结构成分是____ ，在细胞中合成的场所是____ ．

如图所示，体育课上，小丽的铅球成绩为6.4m，她投出的铅球落在