So although Fleming discovered penicillin, it was over a decade ___________someone else turned penicillin into the great drug of the 20th century.

So although Fleming discovered penicillin, it was over a decade ___________someone else turned penicillin into the great drug of the 20th century

1. A.
   when
2. B.
   that
3. C.
   before
4. D.
   unless

So although Fleming discovered penicillin, it was over a decade ___________someone else turned penicillin into the great drug of the 20th century.

   A. when           B. that              C. before             D. unless

So although Fleming discovered penicillin, it was over a decade ___________someone else turned penicillin into the great drug of the 20^th century.

A．when         B．that            C．before          D．unless

So although Fleming discovered penicillin, it was over a decade ___________someone else turned penicillin into the great drug of the 20^th century.

   A. when           B. that              C. before             D. unless

So although Fleming discovered penicillin, it was over a decade ___________someone else turned penicillin into the great drug of the 20th century.