“观察DNA和RNA在细胞中的分布”实验中，正确的实验步骤是（　　）

阅读下面的短文， 按照句子结构的语法性和上下文连贯的要求，在空白处填入一个适当的词，或使用括号中词语的正确形式填空。

    The doorbell rang and Mrs Carson ____ (open) the front door. Her heart sank when she saw Mrs Burbridge. Whenever Mrs Burbridge called, she stayed ____ hours and hours.

    " Good afternoon, Mrs Carson," Mrs Burbridge said, " Do come in." Just as Mrs Carson had feared. Mrs Burbridge stayed for several ____ (hour). It was nearly six o'clock and Mr Carson would be home ____ work soon.

    Mrs Carson kept thinking about ____ she would ask Mrs Burbridge to leave without ____ (make) her angry. "Has your husband got home from work yet?" Mrs Carson asked. "Oh, yes," Mrs Burbidge answered, "he always ____ (get) home at about five o'clock." "It's nearly six o'clock. Won't he be worried about you?" Mrs Carson said.

    "I thought of that," Mrs Burbridge said, "but it's so pleasant here. We've had such a ____ (love) afternoon. You know what I'll do. I'll ring up my husband ____ tell him to come round, too. May I use ____ (you) phone, please?

生物多样性是人类赖以生存和发展的基础，是人类及其子孙后代共有的宝贵财富，下列说法错误的是（   ）

如图，AC=BC,D是AB中点，CE//AB，CE=$\frac{1}{2}$AB.

阅读短文《燕子》，回答问题。
       一身乌黑光亮的羽毛，一对俊俏轻快的翅膀，加上剪刀似的尾巴，这就是活泼机灵的小燕子。
       二三月间，微风轻轻地吹拂着，毛毛细雨从天上洒落下来。千万条柔柳展开了鹅黄色的嫩叶。青的草，绿的叶，各色鲜艳的花，都像赶集似的聚拢来，形成了烂漫无比的春天。小燕子从南方赶来，为春光增添了许多生趣。
       在微风中，在阳光下，燕子斜着身子在天空中掠过，“唧”的一声，已由这边的稻田上，飞到了那边的柳树下了；还有几只横掠过湖面，剪尾或翼尖偶尔沾了一下水面，那小圆晕便一圈一圈地荡漾开去。
       几对燕子飞倦了，落在电线上休息。在蓝天的映衬下，电线杆之间连着的几痕细线，多么像五线谱哇。停着的燕子成了音符，谱出一支春天的赞歌。