Indicate whether each of the following descriptions better applies to B cell receptors (B) or T cell receptors (T). Your answer would be a four-letter string composed of letters B and T only, e.g. BTTT.
( ) They can also be produced as secreted antibodies with the same antigen specificity.
( ) They diversify by class switching and somatic hypermutation.
( ) They often exist as heterodimers.
( ) They have a relatively low antigen-binding affinity.