What type of materials are unsuitable for inspection using MPI?

Magnetic Particle Inspection (MPI) is specifically designed for detecting surface and near-surface discontinuities in ferromagnetic materials, such as iron, cobalt, and nickel. The technique relies on the properties of these materials, which can be magnetized and can retain magnetic fields.

Composites and non-ferrous materials do not possess the magnetic properties that are essential for MPI. Composites, being a combination of materials, often include plastics or other non-ferrous substances that do not respond to magnetic fields. Non-ferrous materials, like aluminum and copper, also do not exhibit the magnetic properties required for inducing magnetic fields and thus cannot be effectively inspected using MPI.

Thus, both composites and non-ferrous materials are unsuitable for inspection using this method, making the choice of both B and C the correct response.