Which of the following statement is correct?
A. Two dissimilar metals are physically joined. Under all circumstances, the one that corrodes is always the more anodic one.
B. Corrosion is always harmful.
C. Two pieces of same metal are joined by a bolt leaving a gap in between. Galvanic corrosion occurs when the metal pieces are immersed in water for weeks.
D. An oxidation reaction occurs only in an environment having oxygen.