30.0 ml of 0.10 M Ca(NO₃)₂ and 15.0 ml of 0.20 M Na₃PO₄ solutions are mixed. After the reaction is complete, which of these ions has the lowest concentration in the final solution?
a. Na⁺
b. NO₃⁻
c. Ca⁺
d. PO₄⁻³