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
What high energy phosphate compound is formed in the citric acid cycle through substrate level phosphorylation: 
Select and explain. 
A. ATP, B. TTP, C. ITP, D. GTP
D: GTP
Answer: D: GTP 
Explanation: None