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
[{'content': 'Which is the whitest colour in vita shade guide? \nSelect and explain. \nA. A1, B. A2, C. B1, D. B2', 'role': 'user'}, {'content': 'Answer: C \nExplanation: The trend is to arrange shade guides in decreasing order of value (lightest to darkest: B1, A1, B2, D2, A2, C1, C2, D4, A3, D3, B3, A3.5, B4, C3, A4, C4). Matching tooth shades is simplified by the arrangement of tabs by value; this arrangement has been found to be easier and more reliable to use.', 'role': 'assistant'}]