In the given scenario where substance A transfers electrons to substance B:
- Substance A is oxidized (loses electrons).
- Substance B is reduced (gains electrons).
Thus, based on the definitions:
- The substance that loses electrons (substance A) acts as the reducing agent.
- The substance that gains electrons (substance B) acts as the oxidizing agent.
Therefore, the correct statement is:
Substance A is oxidized and acts as the reducing agent.