They all appeared this song before.
A. to learn
B. to have learned
C. to not have learned
D. not learning from
网友回答
A. to learn
(答案→)B. to have learned
C. to not have learned
D. not learning from
解析：此处的appeared意为:看来,后可接不定式.根据后面的before得知,这里要用被动结构 to be learned.