1.When using sample data to build the regression model, which of the following about confidence interval for mean y and prediction interval for individual y is/are correct?

a. confidence interval for mean y can be narrower or wider than prediction interval for individual y, given the same value of x.

b. if x is closer to the sample mean x when we construct the confidence interval for mean y, the interval will be narrower.

c. prediction interval for individual y only takes into account of the sampling error.

d. none of the above

2.In simple linear regression, testing for existence of correlation between x and y is equivalent to

A.testing for the existence of slope

B testing for the existence of y-intercept

C.the confidence interval estimate for predicting Y