This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Bile duct, Subject name: Surgery
According to Bismuth classification, type IV cholangiocarcinoma involves: 
Select and explain. 
A. Common hepatic duct, B. Bifurcation only, C. Bifurcation and bilateral secondary intrahepatic duct, D. Bifurcation and unilateral secondary intrahepatic ducts
C: Bifurcation and bilateral secondary intrahepatic duct
Answer: C: Bifurcation and bilateral secondary intrahepatic duct 
Explanation: Bismuth-Corlette Classification of Hilar Cholangiocarcinoma Type Criteria I Tumor confined to hepatic duct, not involving the main biliary confluence II Tumor involving the main biliary confluence but not extending to the right or left ducts IIIa Tumor extending upto the right secondary biliary confluence IIIb Tumor extending upto the left secondary biliary confluence IV Tumor extending bilaterally to the secondary biliary confluence Most common site of cholangiocarcinoma is Hilum