My sister _____ the MP5 for a year. But it broke now.
A：has bought
B：has had
C：buy
下列关于速度的说法正确的是（ ）
What is he busy doing? Doing the experiment ____ (complete) the science project.
设S是数据x1，x2，…，xn的标准差，S1是x1﹣2.5，x2﹣2.5，…xn﹣2.5的标准差，则有（ ）
