To find out how many hours B alone can dig the trench, follow these steps:

1. Determine the Work Rates:
- Let the work rate of A be \(\frac{1}{14}\) of the trench per hour (since A alone can dig it in 14 hours).
- Let the work rate of B be \(\frac{1}{x}\) of the trench per hour (where \(x\) is the number of hours B alone takes to dig the trench).

2. Combined Work Rate of A and B:
- Together, A and B can dig the trench in 10 hours, so their combined work rate is \(\frac{1}{10}\) of the trench per hour.

3. Set Up the Equation:
- The combined work rate is the sum of their individual work rates:
\[
\frac{1}{14} + \frac{1}{x} = \frac{1}{10}
\]

4. Solve for \(x\):
- Rearrange the equation to isolate \(\frac{1}{x}\):
\[
\frac{1}{x} = \frac{1}{10} - \frac{1}{14}
\]
- Find a common denominator (which is 70):
\[
\frac{1}{10} = \frac{7}{70}
\]
\[
\frac{1}{14} = \frac{5}{70}
\]
- Subtract the fractions:
\[
\frac{1}{x} = \frac{7}{70} - \frac{5}{70} = \frac{2}{70} = \frac{1}{35}
\]
- Thus, \(x = 35\).


B alone can dig the trench in 35 hours.

The correct answer is D. 35 hours.