已知R²⁺核内共有N个中子，R的质量数为A，mgR²⁺中含电子的物质的量为

1. A.
   m(A-N)/A mol
2. B.
   m(A-N+2)/A mol
3. C.
   (A-N+2)/Am mol
4. D.
   m(A-N-2)/A mol

A.mol                          B.mol

C.mol                          D.mol

A.m(A-N)/A mol                              B.m(A-N+2)/A mol

C.(A-N+2)/A m mol                        D.m(A-N-2)/A mol

A.m(A-N)/A mol                              B.m(A-N+2)/A mol

C.(A-N+2)/A m mol                        D.m(A-N-2)/A mol