下列符号中，既能表示一个原子，又能表示一种元素，还能表示一种物质的是 (     )

阅读文段，回答下面的问题。

    ①花儿为什么这样红？②还有它生理上的需要。③光波长短不同，产生热效应也不同：红、橙、黄光波长，热效应大；蓝、紫光波短，热效应小。④花的组织，尤其是花瓣，一般都比较柔嫩。⑤野生状态中，在阳光强烈的地方，红、橙、黄花反射了热效应大的长光波，不致引起灼伤，有保护的作用。⑥在树林下、草丛间阳光弱的地方，蓝花反射短光波，吸收微弱的热效应大的长光波，对它的生理作用有利。

根据句子意思，用括号中所给汉语提示或英语单词的适当形式填空。在答题卡标有题号的横线上，写出空缺处所填单词的正确形式。
⑴The ____ will drop below zero. (温度) 
⑵You should understand the____ of learning English well.(重要性)
⑶All of us know bad weather sometimes causes ____ disasters. (自然的)
⑷The ____ girl named Susanne in the world weighs nearly 700 kilograms now. （胖)
⑸Ladies and gentlemen, please let me ____  my best friend to you. (介绍)
⑹____ , he didn't hurt badly in the terrible accident. (lucky)
⑺The ____ are busy fishing in the lake. (fish)
⑻The shoes are ____  to wear, I will not buy them. (comfortable )
⑼Help ____ to some meat, boys and girls. (you)
⑽You had better____  at home until it stops raining. (stay)