the plateau in global warming that took place in the 1950's and 60's is still not well understood; however, evidence from several Notre Dame researches suggest that sunlight- blocking pollution from dirty factories may have played a role, just as natural variability did in ocean circulation.

A understood; however, evidence from several Notre Dame researches suggest that sunlight- blocking pollution from dirty factories may have played a role, just as natural variability did in ocean circulation.
Bunderstood; however, evidence from several Notre Dame researches suggest that sunlight- blocking pollution from dirty factories may have played a role, in the same way that natural variability did in ocean circulation.
C understood, but evidence from several Notre Dame researches suggest that sunlight- blocking pollution from dirty factories may have played a role, as natural variability did in ocean circulation.
D understood, but evidence from several Notre Dame researches suggest that sunlight- blocking pollution from dirty factories may have played a role, as did natural variability in ocean circulation.
E understood, but evidence from several Notre Dame researches suggest that sunlight- blocking pollution from dirty factories may have played a role, as natural variability may have in ocean circulation.