Which of the following statements about the Autocorrelation Function $($ACF$)$ and Partial Autocorrelation Function $($PACF$)$ is TRUE?

a$)$ ACF measures the direct relationship between a variable and its lagged values, while PACF measures the indirect relationship.

b$)$ PACF measures the direct relationship between a variable and its lagged values, while ACF measures the indirect relationship.

c$)$ Both ACF and PACF measure the same relationship between a variable and its lagged values.

d$)$ ACF and PACF are unrelated to each other.