1. a) Let L be the subset of M (IR) consisting of matrices of the form [0 is a subring of M(R). []. Prove that L b) Let U be the subset of M(R) consisting of matrices of the form [ is a subring of M(R). [6]. Prove that U c) Is UUL a subring of M(R)? Prove it or provide a counterexample. d) Find Un L. Is Un La subring of M(R)? Justify your answer. e) Prove the following: If S and T are subrings of a ring R, then SnT is a subring of R.