SAT Question fb56b593: Transitions answer and explanation

English - Transitions - Concession Transition
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Despite its great distance from Earth—it is 570 light-years away—the star Shaula is one of the brightest stars in the sky, ranking 23rd. Although not as bright as Shaula, the star Alkaid also ranks among the 50 brightest stars (40th, to be exact). ______ Alkaid’s brightness is likely due to the star’s relative proximity: Alkaid is only 100 light-years from Earth.
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Difficulty:Medium
Which choice completes the text with the most logical transition?