Which of the following commonly used conversion factors cannot be used to convert length units? 
选项为:A:$\frac{1\text{km}}{1000\text{m}}$  B:$\frac{1\text{Ton}}{2000\text{lb}}$  C:$\frac{1\text{mi}}{1760\text{yd}}$  D:$\frac{1\text{in}}{2.54\text{cm}}$  请回答其中一个选项.