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

小明的解法从第____步开始出现错误；这一步的运算依据应是____．

根据短文理解，从A、B、C、D四个选项中选择最佳答案填空。

    My family spent a few weeks in London last year. We went there in autumn. We think it is the 1 season to visit England. The weather is usually quite good and there are not too many travelers in 2.

    We stayed in a small  3 in the West End. We did most of our sightseeing on foot. We went to look at the places which all travelers would like to 4. We went shopping and spent too much money 5 a lot of things what we liked most was going to the theatres. We didn't have the chance to see such 6 plays at home. A lot of people say English 7 is not so good. We didn't think so. It is true that most of the restaurants are French, Italian or Chinese, but Britain has some very 8 meals.

     In fact, we 9 our holiday so much that we have already decided to 10 there again this year. But we are going to take our umbrellas. I'm sure we'll need them sometimes. It is a really good place to visit.