in the middle of the 1970s
in the late 80s（还是指80年以后？如果是那样，那就是after the 80s）
at the end of the 90s
1970s,1980s,1990s等都可以缩写成70s,80s,90s，还可以加',即1970's
年代前必须加the，如in THE 1970s
1970s即是70s或70's
the mid-1970s,
the late 1980s,
at the end of the 1990s
1970s即为70s或70's
70 era, 80, 90 end of the decade
in the middle of 70s
in the late 80s
at the end of the 90s
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