题目

A man was walking along the street when he saw a woman carrying a large box. It was half in and half out of her car. So the man went up and said, “The box looks very heavy. Let me give you a hand.”“That’s very kind of you,” the woman said, “I’m having a lot of trouble with it. Maybe it sticks (卡住) in the car door.”“Together we’ll soon move it,” the man said. “I’ll get in and take the other end.” And he got in the car.“Right,” the man said. “I’m ready,” and he began to push the box hard.For several minutes, the man and the woman tried to move the box. S下面是描述求一元二次方程ax2+bx+c=0的根的过程的程序框图，请问虚线框内是什么结构？