(1 point) Which of the following subsets of {\mathbb R}^{3\times 3} are subspaces of {\mathbb R}^{3\times 3}? A. The diagonal 3\
times 3 matrices B. The 3\times 3 matrices with all zeros in the third row C. The non-invertible 3\times 3 matrices D. The symmetric 3\times 3 matrices E. The 3\times 3 matrices whose entries are all greater than or equal to 0 F. The 3\times 3 matrices in reduced row-echelon form
