The vertices of a parallelogram are a(x1, y1), b(x2, y2), c(x3, y3), and d(x4, y4). which of the following must be true if parallelogram abcd is proven to be a rectangle?

a- (y4−y3x4−x3=y3−y2x3−x2) and (y4−y3x4−x3×y3−y2x3−x2)=-1

b- (y4−y3x4−x3=y2−y1x2−x1) and (y4−y3x4−x3×y2−y1x2−x1)=-1

c- (y4−y3x4−x3=y2−y1x2−x1) and (y4−y3x4−x3×y3−y2x3−x2)=-1

d- (y4−y3x4−x3=y3−y1x3−x1) and (y4−y3x4−x3×y2−y1x2−x1)=-1