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。
Tom is __________ (缺席) from class today. His mum is ill, he has to look after her at home.
We should pay attention to ________ (grow and improve) a good habit of study.
I received his invitation yesterday, but I didn’t _________ it because I had something more important to do.
Our earth is becoming dirtier and dirtier. We should try our best to reduce the __________(the state of being dirty)   
He will write to me as soon as he __________(到达) Paris.