It took several hundred years paper-making spread to western countries.A. whenB. beforeC.

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A. when
(答案→)B. before
C. after
D. since
解析：连词辨析。     A．当……时    B．在……之前       C．在……之后       D自从……。
句意：造纸术花了几百年才传到西方国家。意思是在造纸术完全传到西方之前用了几百年的时间，故选B。