17. officer, official

officer部队的军官;
official政府官员 eg:an army officer

18. work, job

二者均指工作。work不可数,job可数 eg:a good job

19. couple, pair

couple主要指人或动物;
pair多指由两部分组成的东西 eg:a pair of trousers

20. country, nation, state, land

country侧重指版图,疆域;
nation指人民,国民,民族;
state侧重指政府,政体;
land国土,国家 eg:The whole nation was sad at the news.