当n=k时,左边等于 (k+1)(k+2)…(k+k)=(k+1)(k+2)…(2k),
当n=k+1时,左边等于 (k+2)(k+3)…(k+k)(2k+1)(2k+2),
故从n=k到n=k+1,左边的式子之比是
(k+1)
(2k+1)(2k+2) =
1
2(2k+1) ,
故选B.
当n=k时,左边等于 (k+1)(k+2)…(k+k)=(k+1)(k+2)…(2k),
当n=k+1时,左边等于 (k+2)(k+3)…(k+k)(2k+1)(2k+2),
故从n=k到n=k+1,左边的式子之比是
(k+1)
(2k+1)(2k+2) =
1
2(2k+1) ,
故选B.