Blood flow to the liver is unique in that it receives both oxygenated and (partially) deoxygenated blood. The hepatic portal vein carries venous blooddrained from the spleen, gastrointestinal tract and its associated organs; it supplies approximately 75% of the liver's blood. The hepatic artery provides 30 to 40% of the oxygen to the liver, while only accounting for 25% of the total liver blood flow. 

So, If both a and b given then you should mark both a and b but If it is not given then you should give first preference to hepatic portal vein as it accounts for 75% flow.