Official Magoosh Explanation
Choice (A): Notice that after the "since" clause, we get a participial phrase, "many … moving abroad." This sentence makes the famous missing verb mistake! (A) is incorrect.
Choice (B) is grammatically correct and clear. This is promising.
Choice (C) is logically suspect. We start by talking about "grandchildren living outside the US" and then go on to say "so many have moved abroad"—wait! If they already were living
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