OG详解-OG3 文法 Q19

Choice C is the best answer. The convention being tested is punctuation between a supplementary phrase and a main clause. This choice correctly uses a comma to mark the boundary between the supplementary phrase (“powered…day”), which describes how the LEDs are powered, and the main clause (“the blinking… night”). 
Choice A is incorrect because it fails to mark the boundary between the supplementary phrase and the main clause with appropriate punctuation. Furthermore, placing commas around the phrase “by solar panels” suggests that it could be removed without affecting the coherence of the sentence, which isn’t the case. Choice B is incorrect because it fails to mark the boundary between the supplementary phrase and the main clause with appropriate punctuation. Choice D is incorrect. Placing commas around the phrase “collected by solar panels during the day″ suggests that it could be removed without affecting the coherence of the sentence, which isn’t the case.