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: Dental
Which of the following fuel gases used for soldering, has the highest heat content 
Select and explain. 
A. Hydrogen, B. Natural gas, C. Acetylene, D. Propane
D: Propane
Answer: D: Propane 
Explanation: None