If x1, x2, x3, x4 are four positive real numbers such that
\(x_1 + \frac1{x_2} = 4, \,x_2 + \frac1{x_3} = 1,\, x_3 + \frac1{x_4} = 4, \,x_4 + \frac1{x_1} = 1\) then
a. x1 = x3 and x2 = x4
b. x2 = x4 but x1 ≠ x3
c. x1 x2 = 1, x3 x4 = –1
d. x3 x4 = 1, x, x2 ≠ 1
