The dimension of the null space of A is R^4 because (select all correct answers -- there may be more than one correct answer): A. The basis we found for the null space of A has three vectors. B. rref(A) is the identity matrix. C. rref(A) has three free variable columns. D. Three of the four columns in rref(A) do not have a pivot. E. rref(A) has one free variable column. F. Two of the four columns in rref(A) have pivots. G. rref(A) has a pivot in every ro