Suppose you have three cross-sectional data sets on car prices for the years 2002 and 2004 (one for each of 2002 and 2004) covering the time both before and after the U. S. invasion of Iraq in 2003. You have 90 observations in your data set for 2002 and 120 for 2004. You have decided to pool your 2 cross-sectional data sets together to form one pooled cross-sectional data set. After pooling, you now have total observations in your new pooled cross-sectional data set. What are the primary advantages of pooled cross-sectional data over ordinary cross-sectional data? Check all that apply.

a. Pooling cross sections allows you to form a panel data set.
b. Pooling cross sections allows for a larger sample size.
c. Pooling cross sections allows you to observe changes in key relationships over time.
d. Pooling cross sections allows you to ignore the differences in data over time, since you are left with just one cross-sectional data set.