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.