Succinate dehydrogenase catalyzes the formation of

• A
  $FADH_{2} \longrightarrow FAD$
• B
  $FAD \longrightarrow FADH_{2}$
• C
  $NAD \longrightarrow NADH + H^{+}$
• D
  $NADH + H^{+} \longrightarrow FADH_{2}$