this is a typical "as... as..." structure, not "more... than..." comparison. A&E can be ruled out first. B&D are similarly faulted as they both use "it" to refer to energy, however, the "it" could cause vague interpretation that the energy generated 10 years ago is yet being generated today. as for the phrase "as was the case", i believe i have read about it many times in plenty of readings. so i believe it is a normal usage or an idiomatic phrase as called by ETS. i guess it refers to the meaning stated in the previous sentence as a whole. as you might want to expand the sentence, i'll try as follows: more than ten times as much energy is generated through wind power now as was the case in 1990. more than ten times as much energy is generated through wind power now as the energy generated through wind power in 1990.
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