2.9 Which of the following sets are subspaces of R? a. A= {(1, 1+13,1-13) | 1,4 R} b. B = {(\?, -12,0)|1 R} c. Let be in R. = {(81, 82, 83) R | $1 282 + 33 = y} d. D = {(81, 82, 83) R | $2 Z}