请你根据句意及所给首字母提示，补全单词。
1.We should try our best to protect the e_________.
2.There are four s_________ in a year, spring, summer, autumn and winter.
3.She is a careful girl, so she made few m_________ in the exam.
4.My brother joined the army (军队), because he wanted to be a s_________ in the future.
5.The p_________ of China is over 1.3 billion (十亿).
