问题
单项选择题
If 
of the air in a tank is removed with each stroke of a vacuum pump, what fraction of the original amount of air has been removed after 4 strokes ？（）
A.A
B.B
C.C
D.D
E.E
答案
参考答案：A
解析：
因为每次抽出容器中空气的[*]，所以4次柚出的空气与原始空气的比分别为[*]，其和为：
[*]