With paired data: 

A. The mean of differences is always the same as the difference of the two group means, and the standard deviation of the differences is always the same as the difference of the two group standard deviations. 

B. The mean of differences is always the same as the difference of the two group means, and the standard deviation of the differences tends not to be the same as the difference of the two group standard deviations. 

C. The mean of differences tends not to be the same as the difference of the two group means, and the standard deviation of the differences is always the same as the difference of the two group standard deviations 

D. The mean of differences tends not to be the same as the difference of the two group means, and the standard deviation of the differences tends not to be the same as the difference of the two group standard deviations.