[" 15.Which of the following sets of ions are necessary in "],[" the chemical events for muscle contraction? "],[[" (a) "Na^(+)" and "K^(+),square" (b) "Ca^(++)" and "Mg^(++)],[" (c) "Na^(+)" and "Ca^(++),squarequad (d)Na^(+)" and "Mg^(++)]]