Mark each of the following true or false. 

___ a. M_n(F) has no divisors of 0 for any n and any field F. 

___ b. Every nonzero element of M₂(Z₂) is a unit. 

___ c. End(A) is always a ring with unity ≠ 0 for every abelian group A. 

___ d. End(A) is never a ring with unity ≠ 0 for any abelian group A.

___ e. The subset Iso(A) of End(A), consisting of the isomorphisms of A onto A, forms a subring of End(A) for every abelian group A. 

___ f. R(Z, +) is isomorphic to (Z, +,•)for every commutative ring R with unity. 

___ g. The group ring R G of an abelian group G is a commutative ring for any commutative ring R with unity. 

___ h. The quaternions are a field. 

___ i. (H^*, •) is a group where H^* is the set of nonzero quaternions. 

___ j. No subring of H is a field.