Final answer:
The substances referred to are 'A) Baking soda' for NaHCO3 and 'B) Vinegar' for CH3COOH, which react together in a neutralization reaction.
Step-by-step explanation:
The student is instructed to react 0.21g of NaHCO3 with excess CH3COOH. This corresponds to the substances commonly known as baking soda and vinegar, respectively. When these two react, the baking soda acts as a base and vinegar as an acid, resulting in a neutralization reaction that produces sodium acetate, water, and carbon dioxide gas. NaHCO3 is a key component in baking because it reacts with acidic components to release CO2, helping dough rise. When answered using the options provided, the subject of NaHCO3 is 'A) Baking soda' and CH3COOH is 'B) Vinegar'.