please help po which of the following expressions is equal to -1

a.
[tex] \frac{3m - 1}{1 - 3m} [/tex]
b.
[tex] \frac{5 - 3x}{ - 3x + 5} [/tex]
c.
[tex] \frac{x + y}{y - x} [/tex]
d.
[tex] \frac{x - 1}{x} [/tex]
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