Answer:
A. Exposing a crystal of a semiconductor to heat or light starts displacing valence electrons, which then move throughout the crystal.
Step-by-step explanation:
A semiconductor has several properties, the main two are:
1. Under no special treatment and polarization conditions, there can't be current flow through it due to the high resistance.
2. The semiconductor material can have special change in its structure (excess of electron or holes), to be conductive under certain polarization situations.