This problem and the next use CRSP daily returns. First, get the data and plot the ACF

Question:

This problem and the next use CRSP daily returns. First, get the data and plot the ACF in two ways:
library(Ecdat)
data(CRSPday)
crsp=CRSPday[,7]
acf(crsp)
acf(as.numeric(crsp))
(a) Explain what \lag" means in the two ACF plots. Why does lag differ between the plots?
(b) At what values of lag are there significant autocorrelations in the CRSP returns? For which of these values do you think the statistical significance might be due to chance?