What is the first reaction of the pentose phosphate pathway?

• A. Oxidation of glucose 6-phosphate to 6-phosphoglucono-δ-lactone
• B. Oxidation of 6-phosphogluconate to ketopentose ribulose 5-phosphate
• C. Reduction of 6-phosphoglucono-δ-lactone to glucose 6-phosphate
• D. Reduction of ketopentose ribulose 5-phosphate to 6-phosphogluconate