Which of the following subsets of r3×3r3×3 are subspaces of r3×3r3×3? (r3×3r3×3 denotes the set of all 3×33×3 matrices with real entries.)
a. the 3×33×3 matrices whose entries are all integers
b. the diagonal 3×33×3 matrices
c. the 3×33×3 matrices in reduced row-echelon form
d. the 3×33×3 matrices with all zeros in the third row
e. the invertible 3×33×3 matrices
f. the symmetric 3×33×3 matrices