When a pure semiconductor is heated, its resistance decreases.

When a pure semiconductor is heated, its resistance decreases.

This is because heating a semiconductor provides thermal energy to its electrons, allowing more electrons to break free from their covalent bonds and move into the conduction band. This increases the number of free charge carriers (electrons and holes), which in turn increases the material's electrical conductivity and thus decreases its resistance.

The final answer is $\text{B. decreases}$.