When, in the 1800s, geologists first realized that much of Earth had once been covered by great sheets of ice, some theorized that the phenomenon was cyclical, occurring at regular intervals. Each Ice Age is so destructive, though, that it largely erases the geological evidence of its predecessor. ________ geologists were unable to confirm the theory of cyclical Ice Ages until the 1960s.

Which choice completes the text with the most logical transition?