The theory that the earth's crust and part of the upper mantle are broken into sections that move on a fluid layer of the mantle is a. continental drift b. wegener's theory c. sea floor spreading d. plate tectonics

i think it because equal matrices have equal dimensions, only square matrices can be symmetric. for all indices and . every square diagonal matrix is symmetric, since all off-diagonal elements are zero. in linear algebra, a real symmetric matrix represents a self-adjoint operator over a real inner product space.