For an object to remain at rest, which of the following must be true?

the forces on it are balanced

there is not friction

gravity does not act on it

the onject has no mass

The problem here is that according to aufbau principle before 3d orbital 4s subshell comes.after filling 3p orbital with electrons you have 4 electrons left, you place two in 4s with opposite spin and the other two in 3d in any of its two orbitals of total five orbitals with the same spin.

A.(nh4)(2co3)is the correct answer 

The correct answer is:   bri hope this answer was ! : )