请根据括号中的中文提示、英文释义或句意，写出句中所缺单词，使句子通顺。
（1）We students should do many after-school____ (活动).
（2）People in different countries speak different l____ .
（3）It's ____ (hard or not easy) for me to play basketball well because I can't run fast.
（4）This boy doesn't know the____ (重要性) of study.
（5）L____ , he wasn't badly hurt yesterday.

下图是我国为纪念巴黎公社成立100周年发行的一套邮票。下面有四条发行这套纪念邮票的理由，你认为哪个理由最佳(     )

下列关于亚硝酸盐的叙述错误的是(　　  )

热总是从____处向____处传递。