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 of the following statements is not true about use of silica in the investment materials? \nSelect and explain. \nA. It acts as refractory, B. It regulates thermal expansion, C. Quartz and cristobalite are the allotropic form used in dental investments, D. Beta allotropic form is stable below transition temperature', 'role': 'user'}, {'content': 'Answer: D \nExplanation: Silica exists in at least four allotropic forms: quartz, tridymite, cristobalite, and fused quartz.\xa0\nWhen quartz, tridymite, or cristobalite is heated, a change in crystalline form occurs at a transition temperature characteristic of the particular form of silica. For example, when quartz is heated, it inverts (transforms) reversibly from a “low” room-temperature crystal form, known as α quartz, to a “high” form, called β quartz, at a temperature of 573 °C. This α to β phase transformation is called an inversion.\nThe β-allotropic forms are stable only above the transition temperature noted, and an inversion to the lower α form occurs on cooling in each case.\nRef: Phillip’s 12th edition page 201,202', 'role': 'assistant'}]