Factorize the randomly-given quadratic polynomials
in the form of ax²+bx+c.
因式分解一元二次多項式ax²+bx+c。

For level 1, a=1; for level 2, a=1,2,3,5,7; for level 3, a=1,2,3,..,12.
程度1，a=1；程度2，a=1,2,3,5,7；程度3，a=1,2,3,..,12。