Task: Question Answering
Input (English): Question: In the process of maturation of the affinity of the B lymphocytes in the germinal centers, they act as accessory cells: 
 Options:  <1> Macrophages <2> Th1 lymphocytes. <3> Mastocytes <4> Follicular dendritic cells. <5> Langerhans cells.
Output (English): 4