法兰所用螺栓的类型的材质取决于法兰的（）。
A、公称压力和介质特性
B、公称压力和工作温度
C、法兰直径和介质特性
D、法兰直径和工作温度
参考答案：A
Yesterday ______ 11-year-old boy fell into the river.
Luckily ______ boy was saved by his neighbour at once. [ ]
A. a; a
B. an; a
C. an; the
D. the; a
简述遗赠扶养协议的概念和特征。
