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
The porosity formed due to simultaneous nucleation of solid grains and gas bubbles at the first moment that metal freezes at the mold walls is called: 
Select and explain. 
A. Pinhole porosity, B. Gas inclusion porosity, C. Micro porosity, D. Subsurface porosity.
D: Subsurface porosity.
Answer: D: Subsurface porosity. 
Explanation: None