Which of the following pairs of fractions can be added directly, according to the principle of adding like units?

$\frac{1}{4}$ and $\frac{1}{5}$

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

$\frac{5}{8}$ and $\frac{1}{2}$

$\frac{2}{3}$ and $\frac{3}{4}$

Why can the fractions $\frac{3}{10}$ and $\frac{4}{10}$ be added together directly?

Because their numerators are small.

Because they have a common denominator.

Because they are both less than 1.

Because their sum is less than 1.

Yes, because their numerators are the same.

No, because they do not have a common denominator.