以下哪一种水门汀有遇水凝固加快的特点（）。 A.氧化锌水门汀

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问题单项选择题

以下哪一种水门汀有遇水凝固加快的特点（）。

A.氧化锌水门汀

B.磷酸锌水门汀

C.玻璃离子水门汀

D.聚羧酸锌水门汀

E.氢氧化钙水门汀

答案

参考答案：A

解析：氧化锌丁香酚水门汀的凝固反应必须在有水存在下才能顺利进行，水分越多凝固越快。调和物中含水2%在1天后固化，含水5%可在15分钟内凝固。临床使用时，不必将组织面完全干燥，适当的湿度会有利于水门汀的固化，故可用小棉球蘸水加压成形。

完形填空

As an exchange student, I have been staying in a host family in Beijing for the past five months. The parents are   36  in their mid-30s and the son just turned 5. In exchange for room and board, I teach their son   37 .
Like in many of the other Chinese families, he’s an only child. And though he has   38 plastic trucks and boxes full of crayons, I don’t know   39  he has the time to play. When he’s not in the kindergarten, he’s taking after-school classes. On the weekends, the boy goes to more classes. He has one day off―Sunday.
Compared to his schedule, I feel like my childhood was   40  a waste of time. I learned how to climb trees and draw cartoons. I wasn’t learning Chinese at his age. But he and lots of other children are learning English. Both his parents dream that he will have   41  jobs, higher pay, and bigger houses.
The boy seems to take no interest   42  learning English. He doesn’t need a teacher, but a playmate. He is,   43 , only 5. When I think about my life as a 5-year-old, I know for sure I wasn’t half as   44  as he is.
In the evening, when his parents   45  him to speak with me in English, he gets   46 . He lies on the floors, hides under the table and climbs on the furniture. When he finally stops   47 , and starts talking, it isn’t in English, but it isn’t in Chinese,   48 . He speaks in his own language.
He   49  not be a very good student, but I’m not a great teacher. I just want to be his big brother   50  can speak in gibberish(胡言乱语) with him and help draw trucks, robots or spiders.

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选择题

如图19所示，在水平传送带上有三个质量分别为m、2m、3m的木块1、2、3，

中间分别用原长均为L、劲度系数均为k的相同轻弹簧连接起来，木块与传送带间的动摩擦因数为，现用水平细绳将木块I固定在左边的墙上，传送带按图示方向匀速运动，则（）

A．当三个木块达到平衡后，1、2两木块之间的距离是L+5mg/k

B．如果传送带的速率加倍同，1、2两木块之间的距离将加倍

C．如果将细线剪断，那么木块2的瞬间加速度大小为0

D．2、3间弹簧的弹性势能大于1、2间弹簧的弹性势能

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