Which statement best defines the least common denominator (LCD) for rational expressions?

Factor the quadratic denominator $x^2+7x+12$ into irreducible factors over the integers.

Compute the LCD for the denominators $9x$, $20x^2$, and $14$.

Rewrite the pair of rational expressions $\frac{1}{30x}$ and $\frac{1}{20x^2}$ so both have the LCD $60x^2$.

Two painters are working together to paint a wall. Painter A can paint $\frac{3}{x+2}$ of the wall per hour, while Painter B can paint $\frac{2}{x^2+3x+2}$ of the wall per hour. Determine their combined rate.