Official Explanation Magoosh:
This is a long complicated sentence with a long underlined section. Let's go through the answer choices.
Choice (A) has the subordinate clause ("Although … power"), then the subject ("alternate energy sources"), then some modifiers, then "they would provide". Either the noun "alternate energy sources" or the pronoun "they" could be the subject, but they can't both be the subject. This one commits the double-subject
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