食品保鲜的措施有添加防腐剂、充填气体、放置干燥剂和脱氧剂等。根据物质的性质选用合适的物质用于食品保鲜，对于保证食品安全和人体健康具有重要的意义。

⑴氮气是常用的充填气体。你认为选用氮气的主要原因是_______________。

⑵生石灰是常用的干燥剂。请你用化学方程式表示选用生石灰的原因_____________________。

She was just about to explain ______ she hadn't passed her maths test chiefly out of carelessness ______ her uncle came.

A. her mother when

B. to her mother
why that
C. her mother
that when
D. to her mother
that when