SAT Question 2b08f514: Transitions answer and explanation

English - Transitions - Sequence Transition
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The prime meridian, the global indicator of zero degrees longitude established in 1884, was originally determined using astronomically derived coordinates. ______ as decades passed, new calculations would reveal increasingly precise coordinates, yet the prime meridian remained unchanged; it wasn’t until the 1980s that, spurred by improved geodetic data, the prime meridian was officially moved—roughly one hundred meters east.
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Difficulty:Hard
Which choice completes the text with the most logical transition?