Other radioactive elements ______ since: actinium (Debierne), radiothorium and mesothorium ...

[ Grammar ]

Other radioactive elements ______ since: actinium (Debierne), radiothorium and mesothorium (Hahn), ionium (Boltwood), etc.

A. were discovered

B. are discovered

C. have been discovered

D. had been discovered

Select your answer:

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