某凸透镜的焦距为10cm，当物体沿主光轴从紧贴透镜处向离透镜50cm处移动时，所成像的变化情况中，正确的是(    )

根据短文理解，从A、B、C、D四个选项中选择最佳答案填空。

     I had a wonderful time with my exchangeprogram(交流活动中心) in France. It's much 1 than I thought. I felt a little nervous before I reached France, but there wasn't any reason to be so. My host 2 is really nice.

     They managed various ways to make me 3 at home. Every day I 4 practicing 5 French, I wouldn't believe how 6 progress I 7 in French! Although I still make some mistakes, I can think something in French without

    8  everything into Chinese like I used to. My biggest challenge is learning how to behave at dinner table. All the things are really different 9 the way they are at home. You are not supposed to eat anything with your hands except bread. Another thing is that 10 is very rude to say “I'm full now.” you should say “It is delicious." instead.

$\begin{array}{l}由于a\ne 0,方程a{x}^2+bx+c=0变形为：\\ x^2+\frac{b}{a}x=-\frac{c}{a},\dots \dots 第一步\\ x^2-\frac{b}{a}x+{\left(\frac{b}{2a}\right)}^2=-\frac{c}{a}+{\left(\frac{b}{2a}\right)}^2\dots \dots 第二步\\ {\left(x+\frac{b}{2a}\right)}^2=\frac{b^2-4ac}{4a^2}\dots \dots 第三步\\ \because b^2-4ac>0\\ x+\frac{b}{2a}=\frac{\sqrt{b^2-4ac}}{2a}\dots \dots 第四步\\ x=\frac{-b+\sqrt{b^2-4ac}}{2a}\dots \dots 第五步\end{array}$

小明的解法从第____步开始出现错误；这一步的运算依据应是____．