一个大气质量

经过各方的努力，中国粮食生产连年增长，但是，中国粮食生产正面临着人均耕地不断减少、水资源日益缺乏、农村劳动力持续减少等问题，将来发生减产甚至连年减产的可能性越来越大。这告诉我们（　　）

①要用全面的观点看问题，既要看到成绩，又要看到潜在的危机　

②要正确处理主次矛盾关系，创造有利条件避免矛盾双方转化　

③要具体分析矛盾的特殊性，做到共性与个性具体的历史的统一　

④要承认矛盾，分析矛盾，积极寻找正确的方法解决矛盾

A．①②

B．①④

C．②③

D．③④

经济发展

砌体结构当静力计算为弹性方案时，其含义哪一种说法是正确的？（）

A.对墙而言，楼盖和屋盖可视为铰接的水平杆

B.楼盖各点的水平位移是相等的

C.楼盖在平面外的刚度可视为无穷大

D.楼盖在其平面内的刚度可视为无穷大

An analog signal’s characteristic quantity representing data may (66) . at any instant, any value within a continuous (67) . For example, an analog signal may follow continuously the values of another physical quantity representing data.

A discrete signal is composed of (68) elements in time, each element having one or more characteristic quantities which can represent data. Examples of characteristic quantities are amplitude, waveform, (69) , and position in time. A digital signal is a special case of discrete signals, in which data are represented by a (70) number of well-defined discrete values that one of its characteristic quantities may take in time.

（69）处填（）。

A．duration

B．period

C．persistence

D．span

业主公约是一种公共契约，属于协议、合约的性质。它是由全体业主承诺共同订立，对全体业主(包括物业使用人、继受人)有普遍约束力的自律性规范。 ( )