Calculate the coordinates of the midpoint of the line segment AB, when

a) A [[$ = (1, 3) $]] and B [[$= (3, 1)$]].

b) A [[$= (4, 8) $]] and B [[$= (2, 2)$]].

c) A [[$= (-4, 9) $]] and B [[$= (-6, 1)$]].

d) A [[$= (-5, -4) $]] and B [[$= (-1, -2)$]].