Answer:
C. It is made from three layers of semiconductors.
Explanation:
Bipolar transistors (pnp and npn) are usually made from a sandwich of three semiconductors layers. The main charge carriers are holes and electrons. An npn transistor has two semiconductor junctions that share a thin middle positively enhanced region (the material is enhanced with impurities rich in holes which are the positive charge carriers), and a pnp transistor has two semiconductor junctions that share a thin middle negatively enhanced region (material is enhanced with impurities rich in electrons, the negative charge carriers).
