Which choice completes the text so that it conforms to the conventions of Standard English? In 1881, French chemist Camille Faure redesigned the rechargeable lead-acid battery. Faure's design greatly increased the amount of electricity that the original battery, which the French physicist Gaston Planté ______blank fifteen years earlier, could hold.

Choice D is the best answer. Faure redesigned the battery in 1881, and the original battery was invented 'fifteen years earlier.' Notice that this is the only choice in the past tense. To indicate that a past occurrence took place before another past occurrence, we need to use 'had' + the past tense form of the verb. This is called the 'past perfect' tense. Choice A is incorrect. This choice creates a tense error. Planté invented the original battery in the past, so we shouldn't use the present tense 'is inventing.' Choice B is incorrect. This choice creates a tense error. Planté invented the original battery in the past, so we shouldn't use the future tense 'will invent.' Choice C is incorrect. This choice creates a tense error. Planté invented the original battery in the past, so we shouldn't use the present tense 'invents.'