阅读下面材料，在空白处填入适当的内容（不多于3个单词）或括号内单词的正确形式。

Once there was a free bird. She floated in the sky,____(catch) worms for lunch and swam in the summer rain trickles(滴), like many ____birds. But she had a habit:____some event occurred in her life, whether good or bad, the bird would pick up a stone ____the ground to memorize it. Every day she sorted out her stones, laughed remembering joyful events, and cried remembering the sad ones.

The bird always took the stones with her, whether she was flying in the sky or walking on ____earth, and she never forgot about them. Years had passed ____the free bird got a lot of stones, but she still kept on sorting them out, remembering the past. It was becoming more and more difficult for her to fly.

The bird was unable to move on her own，____could she catch worms anymore. Only rare rain gave her the necessary energy.But the bird ____(brave) endured all the hardships, ____(guard) her precious memories. After some time the bird died of ____ (starve) and thirst. 

根据句意及首字母提示完成单词。

RNA是生物体内最重要的物质基础之一，它与DNA、蛋白质一起构成了生命的框架．近年科学家发现，一些RNA小片段能够使特定的植物基因处于关闭状态，这种现象被称作RNA干扰（RNAInterference，简称RNAi）．近日，分子生物学家发现RNAi在老鼠和人体细胞中也可以“停止基因活动”．根据上述材料回答：

（1）老鼠细胞的细胞核、____ 、____ 、细胞质基质等结构中有RNA的分布．

（2）RNAi使基因处于关闭状态，是遗传信息传递中的____ 过程受阻，遗传信息不能以蛋白质的形式表达出来．如胰岛A细胞有与合成胰岛素相关的基因也有与合成胰高血糖素相关的基因，但只能产生____ ，结合以上材料，分析其可能原因是：____．

（3）过去的很长时间内，科学家一直认为，RNA仅仅是传递遗传信息的信使．列举一例，说明“RNA也可以是控制生物的性状的遗传物质”：____ 

18的 $\frac{1}{3}$ 是____ ；12的 $\frac{3}{4}$ 是____ 。

A.3   B.6   C.9  D.12