到了20世纪70年代，人们又对太阳能产生了兴趣。
Then in the 1970s, there was an interest in solar energy again.
在20世纪90年代，那里产出的电能大约能满足旧金山一半的用电需求。
In the 1990s enough electricity to meet about half the needs of San Francisco was being generated there.
跟踪北极能种群的现代方法仅在20世纪80年代中期才被采用，而且，要在大范围内连续采用这种方法，其代价非常高昂。
Modern methods for tracking polar bear populations have been employed only since the mid-1980s, and are expensive to perform consistently over a large area.
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