选项
A.
A.${left[ {{{left( {frac{{k - 1}}{{k + 1}}} right)}^2} - 1} right]^{1/2}}b$
B.
B.${left[ {{{left( {frac{{k + 1}}{{k - 1}}} right)}^2} - 1} right]^{1/2}}b$
C.
C.${left[ {{{left( {frac{{k - 1}}{{k + 1}}} right)}^2} + 1} right]^{1/2}}b$
D.
D.${left[ {{{left( {frac{{k + 1}}{{k - 1}}} right)}^2} + 1} right]^{1/2}}b$
正确答案：