年金现值系数
参考答案：
[1-（1+i）^-n]/i称为年金现值系数，记作PVIFA（i，n），或者（PA/A，i，n）。
I had wanted to help you last night but I couldn't spare any time, for I ________ a composition which I
have to hand in this morning.[ ]
A. wrote
B. was writing
C. had written
D. have written
阅读《前赤壁赋》中的一段文字，回答问题：
客有吹洞箫者，倚歌而和之。其声呜呜然，如怨如慕，如泣如诉；余音袅袅，不绝如缕。舞幽壑之潜蛟，泣孤舟之嫠妇。
箫声描写中注入了怎样的情感基调？与文章的下文有何联系？
time, for I ________ a composition which I