[ 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
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Present and Past Tenses › ViewI … for two hours before I twisted my ankle.
A. climbed
B. had been climbing
C. had climbed
D. was climbing
General Knowledge › View
Which is the smallest ocean in the world?
A. Atlantic Ocean
B. Gold Ocean
C. English Ocean
D. Tiger Ocean
E. Arctic Ocean