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
In humans and other primates, as well as in guinea pigs, this vitamin cannot be synthesized, because of the absence of the enzyme L-gulonolactone oxidase; the vitamin is: 
Select and explain. 
A. Pantothenic acid, B. Nicotinic acid, C. Ascorbic acid, D. Folic acid
C: Ascorbic acid
Answer: C: Ascorbic acid 
Explanation: None