This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Biochemistry
Enzymes are usually: 
Select and explain. 
A. Lipid, B. Nucleic acids, C. Carbohydrates, D. Protein
D: Protein
Answer: D: Protein 
Explanation: Enzymes are mainly proteins, that catalyze (i.e., increase the rates of) chemical reactions with the exception of catalytic RNA molecules, or ribozymes.In enzymatic reactions, the molecules at the beginning of the process are called substrates, and the enzyme conves them into different molecules, called the products.