Person Sacrifice

3 people need to cross a bridge. It is pitch dark and the bridge itself is rickety, can only hold two at a time, and can only hold 100 pounds. More than that, you fall to your death in the lake full of crocodiles and alligators. You are A, and you weigh 79 pounds. B weighs 21 pounds, and C weighs 99 pounds. You need light, and you have a flashlight. One person has to volunteer to be left behind for the night, because the person with the flashlight has to go back to give it to another person. Who has to be sacrificed?