节理按其力学成因，一般分为哪两类？请说出剪节理的特征。

对下列实验过程的评价，正确的是（　　）

A．向某固体中滴加稀盐酸，有气泡产生，该固体一定是碳酸盐

B．在无色溶液中滴入酚酞试液，溶液变红色，该溶液一定是碱溶液

C．生石灰块投入水中，水会沸腾，生石灰变成熟石灰的过程是放热反应

D．中和反应生成盐和水，因此生成盐和水的反应一定是中和反应

一、根据下列资料回答第86-90题。某省城乡居民最低生活保障及社会保障体系的有关情况如下：以医疗保险为例，2004年，该省城镇社区的服务设施数为7446个，还有22321个便民、利民服务网点，而农村社会保障体系点只有734个；在最低生活保障方面，城镇低保人数为99.04万人，而农村只有26. 81万人；低保金额城镇为6648.5万元，农村仅为3681.2万元。以卫生机构人员、床位、医院数为例，2004年该省医院数660个，其中市医院数为447个，县医院数为213个；医院床位数为77773个，其中市医院床位数为53830个，县医院床位数为23943个；医院职工人数为105703人，其中市医院为71870人，县医院为33833人。

以下判断正确的是()。

A．农村基层的看病难问题已得到一定缓解

B．市医院床位数占全省医院床位数的三分之二强

C．卫生医疗等公共资源正向县级医院倾斜

D．市医院职工人数平均规模明显高于县医院

由一根内壁光滑的玻璃管构成一个直角三角形处于竖直平面内，倾斜角为θ=37°，让两个相同的小球同时从顶端A静止开始出发．一个球沿AC做加速度大小为a=gsin37°的匀加速运动至C，到达C所用的时间为t₁，另一个球竖直自由下落经过B后匀速运动到达C，所用的时间为t₂，在转弯处有个极小的光滑圆弧，可确保小球转弯时速度大小不变，且转弯时间可以忽略不计．（其中sin37°=0.6，cos37°=0.8）．比较t₁与t₂的关系（　　）

A．t₁＜t₂

B．t₁=t₂

C．t₁＞t₂

D．条件不够无法确定

The great advance (91) rocket theory 40 years ago showed that liquid-fuel rockets were far (92) in every respect to the skyrocket with its weak solid fuel, the only kind of rocket then known. (93) , during the last decade, large solid-fuel rockets with (94) fuels about as powerful as liquid fuels have (95) their appearance, and it is a favorite layperson’s question to (96) which one is "better". The question is meaningless. (97) might as well ask whether a gasoline or a diesel engine is "better". It all (98) on the purpose. A liquid-fuel rocket is complicated, (99) has the advantage that it can be controlled beautifully. The burning of the rocket engine can be stopped completely; it can be reignited when (100) . (101) , the thrust can be made to vary by adjusting the speed of the fuel pumps. A solid-fuel rocket, on the other hand, is rather (102) in construction, (103) hard to build when a really (104) size is desired. But once you have a solid-fuel rocket, it is ready for action (105) very short notice. A liquid-fuel rocket has to be fueled first and cannot be (106) readiness for very long after it has been fueled. (107) , once a solid-fuel rocket has been ignited, it will keep burning. It cannot be stopped and reignited (108) desired (it could conceivably be stopped and reignited after a pre-calculated time of burning has elapsed, and its thrust cannot be varied. Because a solid-fuel rocket can be kept ready for a long time, most military missiles (109) solid fuels, but human-piloted spaceflight needs the fine adjustments that can (110) be provided by liquid fuels.

A．stayed

B．controlled

C．detained

D．held

自然界种群进化的主要动力是遗传变异和______。