The verticles of parallelogram ABCD are located at points A(-2,-1), B(6,1), C(10,7), D(2,5). Which of the following statements are true?

a > (6,6) is the midpoint of CD
b > (2,0) is the midpoint of AD
c > (4,8) is the midpoint of BC
d > (0,2) is the midpoint of AB