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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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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