(5 points) Which of the following sets are subspaces of R? OA. {(-6x, 9x, 7x) Ix arbitrary number } B. ((4, y, z) I y, d arbitrary numbers} C. (x, y, z) I 4x- 2y - 3z 0} D. ((x, y, z) Ix+y+z 6} E. ((x, y, z) | - 4x + 2y = 0, 3x 6z = 0} OF. (x, y, z) I x, y, z > 0}