6. When Samuel is doing protein purification for a mixture of three proteins (protein A with pI 4, protein B with pI 9, protein C with pI 7), which statement in the following is correct ? (A) When the mixture is adjusted to pH 6, only protein A can bind in an anion exchange column. (B) When the mixture is adjusted to pH 8, protein B can not bind in a cation exchange column. (C) When the mixture is adjusted to pH 10, the elution sequence in a gel filtration column is A-C-B. (D) When the mixture is adjusted to pH 8, only protein B can bind in an anion exchange column. (E) When the mixture is adjusted to pH 10, all three proteins can not bind in an anion exchange column.