  We address two important issues that arise in recent studies of iron-based
superconductivity. (1) Why are the T_c of A_xFe_{2-y}Se_2 and the single unit
cell FeSe on SrTiO$_3$ so high despite both only have electron pockets? (2)
What (if any) are the effects of orbital order and orbital fluctuation on the
Cooper pairing. Our conclusions are summarized in the third paragraph of the
paper.
