根据所给句子和汉语提示写出恰当的单词，每空一词。
Protecting the ________________(环境) is our duty.
Leo couldn’t stop __________________(思念) his parents when studying abroad.
My mother took me to school when I was at ________________(小学) school.
A sparrow was ______________(死). Let’s bury(埋) it under the tree.
Why not take a ___________(直达) flight to Sydney? It is much faster.