对于石膏类模型材料，其混水率越大，则（）
A.凝固时间越长，强度越高
B.凝固时间越短，强度越低
C.凝固时间越长，强度越低
D.凝固时间越短，孔隙越多
E.孔隙越少，强度越高
参考答案：C
解析：半水硫酸钙与水的比例可用混水率来表示。混水率（W／P）是水的体积除以半水硫酸钙粉末重量所得的分数。如100g熟石膏与60ml的水混合时，混水率为0.6。实践证明，石膏模型材料混水率越大，凝固时间越长，最后的生成物越脆，强度越低。这是由于混水率大，材料的结构疏松，水量增加，二水硫酸钙的结晶核减少，结晶体间的相互胶结现象也少，使材料强度降低，同时当多余的水挥发后，会形成一些微小的孔隙，称为石膏的孔隙率。混水率、孔隙率和材料强度之间的关系是，混水率越高，孔隙越多，材料强度越低。
Honesty, my mum always used to tell me, is the best policy. Of course, this didin’t include her when she told me that if I didn’t eat all my vegetables Father Christmas would find out and wouldn’t give me any presents. But when it comes to medicine, I had assumed it was important to always be honest with my patients. After all, the doctor patient relationship is based on trust, and therefore honesty is essential, Or so I thought. I had just started working in geriatrics(老年病科). Mr. McMahon was brought in when his baby was faound very swollen. I took a medical history from his daughter would accompanied him in the ambulance. She’d been his main carer for years. I stood looking at him as she gave a detailed history. “Has he lost any weight recently?” I asked, “Well, it’s funny you should mention that, but yes.” She said slowly. There was silence for a few moments. “Why? What are you worried about?” she asked, I hesitated. She was obviously very involved in his care and it was only fair that I told her the truth. “Well, we need to prove it’s not cancer.” I said and talked briefly about some of the tests I was going to order. Half an hour later, a nurse called me: “Mr. McMahon’s daughter broke down－she said you told her he had cancer.” My heart sank. By the time I arrived at the ward, my consultant was already there, explaining that we still had to run lots of tests and that it was by no means confirmed that he had cancer. I stood silently at the end of the bed. My consultant was obviously angry with me and as we left Mr. McMahon, she turned to me. “Why on carch did you do that?” she asked to disbelief. I looked at her andbit my hp. “She asked me what I was worried about and I told her.” I said, hanging my head. “And give her more to worry about?” replied my consultant. “You don’t say the word ‘cancer’ until it’s confirmed. Even if you suspect it, think very carefully before you tell people.” As it turned out, it wasn’t cancer. But I did learn that when somecone is stressed and worried about their loved one they’re sometimes selective in what they hear－and as a doctor it’s important to be mindful of this. In being truthful, I’d made the situation worse. 小题1:.The purpose of the first two paragraphs is to show that the author .
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