SAT Question e6b1e12c: Transitions answer and explanation

English - Transitions - Contrast Transition
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Long thought to be sessile (immobile), adult Chelonibia testudinaria, barnacles that adhere to sea turtle shells, have been observed to shift slightly in position over time—a phenomenon that has been attributed to the barnacles’ passive displacement by water currents. ______ a research team found that adult C. testudinaria moved toward the heads of their sea turtle hosts and thus against the prevailing water flow, behavior consistent with self-initiated locomotion.
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Difficulty:Hard
Which choice completes the text with the most logical transition?