Although everything seems to have been taken into consideration，       accident can happen when the astronauts walk in            space．

A．不填；the

B．an；the

C．the；the

D．an；不填

化学用语是学习化学的基本工具，请用化学符号或化学式填空：

（1）3个硝酸根离子_____________

（2）2个甲烷分子__________

（3）葡萄糖分子________