问题补充：
Though I have often heard this song . I have never heard you it.
A. being sung, sang
B. sang, singing
C. sung, sing
D. to be sung, to sing
答案：
A. being sung, sang
B. sang, singing
(答案→)C. sung, sing
D. to be sung, to sing
解析：第一空填 sung，因为 song 与 sing 是被动关系；第二空填 sing，因为 you 与 sing 是主动关系。
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