Although photovoltaic cells are designed to absorb sunlight to generate electricity ______ their efficiency drops significantly when they are covered by dust or heavy snow.
Which choice completes the text so that it conforms to the conventions of Standard English?
electricity but
electricity
electricity,
electricity, but
Correct Answer: C
Choice C is the best answer. It correctly uses a comma to separate the introductory dependent clause ("Although...electricity") from the main independent clause ("their efficiency...snow").
Choices A and D are incorrect because they include the coordinating conjunction "but." Since the sentence begins with the subordinating conjunction "Although," adding "but" creates a redundant double contrast. Choice B is incorrect because it omits the necessary punctuation after the introductory dependent clause.