For 13-year-old Arrianna Merritt, the words mother and hero go hand in hand. Three years ago Sunday Arrianna watched her mother Kim Dahl drive a school bus loaded with children across the Mississippi River when the bridge began to buckle up and down. Thirteen people were killed in the bridge collapse in Minneapolis, Minnesota. What Arrianna remembers most was the choice her mother made that day after the bus became tilted upward on a steep incline. Either you let us free fall -- or you hold that brake to the ground and you stay where you are. Dahl held the brake with her foot until she could reach for the parking brake. Millions of viewers saw the jaw-dropping images of the school bus as it teetered on the edge of a broken stretch of highway, with nothing but a guard rail separating it from a deadly fall. Arrianna sat in the bus with her brother, 50 other children and eight adults. The bus went up, and then it went down, Dahl recalls, adding it soon turned into free fall and she was hanging on for dear life. I thought this is it,I 'm not getting out, and I 'm going to die. The new 35W bridge: Is it the safest in America ? With the parking brake on, Dahl kept the bus in place while everyone made it out the rear emergency door safely. Arrianna, 10 at the time, and her then-5-year-old brother David, were the last ones and had refused to get off with the others. We did n't