many times, he finally understood it. A. ToldB. TellingC. Having told D. Having been told

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A. Told
B. Telling
C. Having told
(答案→)D. Having been told
解析：本题考查非谓语的用法。tell与主语之间的关系是被动和完成的关系，所以用having been done来表示被动和完成。