From the following subsets of r³x3, select all that are subspaces r³x3. There may be more than one correct answer.
1) The set of all 3x3 matrices with real entries
2) The set of all 3x3 matrices with determinant equal to zero
3) The set of all 3x3 matrices with rank less than or equal to 2
4) The set of all 3x3 matrices with trace equal to zero