Explain why glutamic acid (pI = 3.08) elutes before threonine (pI = 5.60)  when their solution having pH 5 is passed through a cation-exchange column.

• A. Because glutamic acid has a higher amount of negative charge at this pH due to its lower pI value.
• B. Because glutamic acid has a higher amount of positive charge at this pH due to its lower pI value.
• C. Because threonine has a higher amount of negative charge at this pH due to its higher pI value.
• D. Because threonine has a higher amount of positive charge at this pH due to its lower pI value.

Answer: (A)