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.${left[ {{{left( {frac{{k + 1}}{{k - 1}}} right)}^2} + 1} right]^{1/2}}b$
