Although for many years, the two old sisters recognized each other at once.
A. having been separated
B. being separated
C. having separated
D. had been separated
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(答案→)A. having been separated
B. being separated
C. having separated
D. had been separated
解析：A考查非谓语动词用法。前后主语一致并且前边动作发生在后面的动作之前，故用现在分词的现在完成时。