At a point in a two dimensional stress system, the normal stress on two mutually perpendicular planes are σxx and σyy and shear stress is τxy. One of the principal stresses will become zero when the value of shear stress τxy is

• A. ± (σxxσyy)
• B. \( \pm \sqrt {{\sigma _{xx}} - {\sigma _{yy}}} \)
• C. \( \pm \sqrt {{\sigma _{xx}} + {\sigma _{yy}}} \)
• D. \( \pm \sqrt {{\sigma _{xx}}{\sigma _{yy}}} \)