回流冷凝器置于塔顶，回流液借重力回流到塔内称为（）。
A、强制回流
B、自然回流
C、内回流
D、冷回流
参考答案：B
The following data pertains to an investor's stock: The stock will pay no dividends for two years. The dividend three years from now is expected to be $1. Dividends are expected to grow at a 7% rate from that point onward. If the investor requires a 17 percent return on their investments, how much will the investor be willing to pay for this stock now（）
A. $6.24.
B. $7.31.
C. $8.26.
对于记录型信号量，在执行一次wait操作时，信号量的值应当 A ；当其值为 B 时，进程应阻塞。在执行signal操作时，信号量的值应当 C ；当其值为 D 时，应唤醒阻塞队列中的进程。 A，C：(1)不变 (2)加1 (3)减1 (4)加指定数值 (5)减指定数值 B，D：(1)大于0 (2)小于0 (3)大于等于0 (4)小于等于0
