So although Fleming discovered penicillin, it was over a decade ___________someone else turned penicillin into the great drug of the 20th centuryA.whenB.thatC.beforeD.unless
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C解析考查时间状语从句。句意“尽管是弗莱明发现的青霉素，但是青霉素广泛用于药物是几十年后的事情。”it is +一段时间+before意为“过了多久才......”。故选C。