Pick out the wrong statement.

A. Seam welding, projection welding & spot welding are the classification of electrical resistance welding

B. Electrode tip in spot welding is of copper and the tip diameter should be equal to t (where, t = plate thickness to be welded)

C. In spot welding, two pieces to be joined are overlapped and placed between two electrodes

D. Mild steel sheet cannot be spot welded

Please do not use chat terms. Example: avoid using "grt" instead of "great".