净现值
参考答案：
是指方案在寿命期内各年的净现金流量，按照一定的折现率折现到期初时的现值之和。
加里东构造阶段，华北、塔里木板块与扬子板块以（）洋相隔
When you are firing a pyrotechnic distress signal，it should be aimed（）.
A.horizontally and directly abeam of your vessel
B.at the vessel whose attention you want to attract
C.into the wind
D.at greater than 60 degrees above the horizon
