A：极性键
B：非极性键
C：共价键
D：氢键
中科院院士袁隆平在培育超级杂交水稻时，专门挑选叶片直、窄、厚的水稻植株，其目的是____。
金刚石、石墨和木炭都是由碳元素组成的单质。下列叙述正确的是（ ）
根据句意和首字母提示，完成句中所缺单词①We should brush our____ (tooth) every day.②—What’s Jane doing? —She’s b____ her sister.③ What is she doing for v____ ?④—How do you come to school? —I usually ride a b____ to school.⑤ I want to s ____ an e-mail to my aunt.
