During pancreatic development, which of the following transcription factors plays a crucial role in the specification of insulin-producing beta cells, somatostatin-producing delta cells, and glucagon-producing alpha cells?
A. PAX 4 and PAX 6
B. PDX1 (Pancreatic and duodenal homeobox 1)
C. NGN3 (Neurogenin 3)
D. NKX2.2 (NK2 homeobox 2)