Haemoglobin is made up of two $\alpha$ chains and two $\beta$ chains. In about $5\%$ of all humans, one amino acid in the $\beta$ chain differs from the normal form. Which level(s) of protein structure may be altered in these humans?
- Aprimary only
- Bprimary and quaternary only
- Cprimary, secondary and tertiary only
- Dquaternary only