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': 'When making a polysulphide impression for a caste crown, custom trays are usually preferred over stock trays for all of the following reasons except \nSelect and explain. \nA. Custom trays facilitate uniform contraction of impression material, B. Custom trays require less impression material, C. Stock trays may be short in the flange area, D. Impressions made in the custom trays are easier to remove from the mouth', 'role': 'user'}, {'content': 'Answer: D \nExplanation: The use of a custom tray is recommended to reduce the quantity of material required to make impressions; thus, any dimensional changes attributed to the materials are minimized. A custom tray allows a uniform distribution of impression material between the tray and the object, which also improves accuracy. This is especially true for polysulfide impression\xa0 material.\xa0\nThe use of custom\xa0 trays\xa0 for\xa0 polyether and addition silicone impressions is not critical, since these\xa0materials are stiffer and have less polymerization shrinkage than the polysulfide material. In addition, disposable stock\xa0trays work satisfactorily. Note that the use of less material in a custom tray reduces the compressibility of\xa0the impression, which can make removal of the impression tray more difficult. When severe undercuts are present, the use of a custom tray should be avoided. The stock tray should\xa0be rigid, thereby minimizing flexure of the tray during impression making.\nPhillips 12 ed page no 156', 'role': 'assistant'}]