What is the bond order of the $Be_{2}$ molecule?

According to Molecular Orbital Theory,which of the following statements is/are correct?
$(A)$ $C_2^{2-}$ is expected to be diamagnetic
$(B)$ $O_2^{2+}$ is expected to have a longer bond length than $O_2$
$(C)$ $N_2^{+}$ and $N_2^{-}$ have the same bond order
$(D)$ $He_2^{+}$ has the same energy as two isolated $He$ atoms

Which of the following options represents the correct bond order?

Between $1s-1s$ and $1s-2s$ combinations,which one does not form a molecular orbital? Why?

Use molecular orbital theory to explain why the $Be_{2}$ molecule does not exist.

Which of the following species,$O_2$,$O_2^+$,and $O_2^-$,has the lowest bond order?