what is the ground state electronic configuration of a `_(27)` CO atom in the gas phase ?
A. `1s^(2)2s^(2)2p^(6)3s^(2)3p^(6)3d^(7)4s^(2)`
B. `1s^(2)2s^(2)2p^(6)3s^(6)3d^(9)`
C. `1s^(2)2s^(2)2p^(6)3s^(6)3d^(8)4s^(1)`
D. `1s^(2)2s^(2)2p^(6)3s^(6)3d^(7)4s^(2)`
