in the 1940s 与in the 1940's的区别?为什么My brother served in the army in the 1940's.这一句子中要用in the 1940‘s而不用in the 1940s?

in the 1960's 与 in the 1960s 区别 in the 1940s 与in the 1940's的区别?为什么My brother served in the army in the 1940's.这一句子中要用in the 1940‘s而不用in the 1940s? in the barber与in the barber's用法区别 in 1980's 与 in the 1980's的区别 In the 90s 与In 90s 的区别 in 1950s 与in the 1950s有什么区别?in 1950s是表示1950----1959还是1940---1949?in the 1950s呢? in the cat,s In the stationary’s in the 1940's in 1940s in the 1940s区别~ in the 1980s 与 in 1980的区别 in the in the 1720's呢? in 1980/in 1980s / in the 1980 / in the 1980s / in the 1980's/in the 1980' in the 1980s与in the 1980's一样吗 in the 1660s与in the 1660‘s的区别 in the 1970与 in the 1970s 有什么区别吗?in the 1970 与 in the 1970s 有什么区别吗? in the 1970s；in 1970s；in 1970's；in the 1970's哪个对? in the 1940's和in the1940s一样吗? in 1970、in the 1970、in the 1970‘s、in the 1970s有什么区别