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
Greening refers to: 
Select and explain. 
A. Greenish discolouration of porcelain due to escaping of silver vapours, B. Also known as wet strength of gypsum product, C. Observed in silica bonded investments due to drying of colloidal silica gel, D. All of the above
A: Greenish discolouration of porcelain due to escaping of silver vapours
Answer: A: Greenish discolouration of porcelain due to escaping of silver vapours 
Explanation: Discoloration of the porcelain near the cervical region of the metal-ceramic prosthesis has been reported to occur when a silver-containing alloy is used as the substrate. 
Color changes included green, yellow-green, yellow-orange, orange and brown hues. 
This discoloration phenomenon has generally been called “greening.”

Ref: Phillip’s Science of Dental Materials ed 12 pg 380