<p>In balancing the half reaction&nbsp;<math xmlns="http://www.w3.org/1998/Math/MathML" class="wrs_chemistry"><msub><mi mathvariant="normal">S</mi><mn>2</mn></msub><msubsup><mi mathvariant="normal">O</mi><mn>3</mn><mrow><mn>2</mn><mo>-</mo></mrow></msubsup><mo>&#160;</mo><mo>&#8594;</mo><mi mathvariant="normal">S</mi><mo>(</mo><mi mathvariant="normal">s</mi><mo>)</mo></math>, the number of electrons that must be added is:</p>