下列属于管壳式换热器的是（）。
A、套管式换热器
B、固定管板式换热器
C、浮头式换热器
D、U型管式换热器
参考答案：B, C, D
拍卖可以是单向的也可以是双向的（）
For many years, the principle goal of computer (73) was to write short pieces of code that would execute quickly. The (74) needed to be small, because memory was expensive, and it needed to be fast ,because processing power was also expensive.
A. users B. programmers C. computer D. program
