The equations for principal stresses are valid only when
(a)σ x and σ y are both tensile
(b) σ x is compressive and σ y is tensile
(c) σ x is tensile and σ y is compressive
(d) None
(a)σ x and σ y are both tensile
(b) σ x is compressive and σ y is tensile
(c) σ x is tensile and σ y is compressive
(d) None