The two ions responsible for hardness in water are

• A Ca\(^{2+}\) and / or Mg\(^{2+}\)
• B K\(^+\) and / or Mg\(^{2+}\)
• C Ca\(^{2+}\) and / or Li\(^+\)
• D Ca\(^{2+}\) and / or Na\(^+\)