问题
单项选择题
The sample of per square foot sales for 100 U. S. retailers in December 2004 is an example of:（）
A. cross -sectional data.
B. unbiased data.
C. time -series data.
答案
参考答案：A
解析：
Cross - sectional data are a sample of observations taken at a single point in time. A time - series is a sample of observations taken at specific and equally spaced points in time.