Although Fleming discovered Penicillin, it was over a decade someone else turned it into the great drug of the 20th century.
A. before
B. since
C. when
D. as
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(答案→)A. before
B. since
C. when
D. as
解析：考查固定句式：It was/will be+一段时间+before…“要过多长时间就/才…”,句意：虽然弗莱明发现青霉素，但是10多年后其他人才把它变成20世纪伟大的药物。选A。