For which pair of fractions can the larger denominator be used as the least common denominator?

$\frac{1}{3}$ and $\frac{2}{5}$

$\frac{5}{6}$ and $\frac{7}{18}$

$\frac{3}{4}$ and $\frac{5}{6}$

$\frac{2}{7}$ and $\frac{3}{8}$

When finding a common denominator for $\frac{1}{4}$ and $\frac{9}{20}$, why is $20$ the best choice?

Because $20$ is larger than $4$.

Because $20$ is an even number.

Because $20$ is a multiple of $4$.

Because $20$ is the product of $4$ and $9$.