Etermine which of the following subsets of R3×3 are subspaces of R3×3. 1. The singular 3×3 matrices 2. The 3×3 matrices whose entries are all integers 3. The symmetric 3×3 matrices 4. The 3×3 matrices with all zeros in the first row 5. The invertible 3×3 matrices 6. The 3×3 matrices with trace 0 (the trace of a matrix is the sum of its diagonal entries) 7. The upper triangular 3×3 matrices 8. The 3×3 matrices in reduced row-echelon form