In the following graph, the magnitude of concentration difference across the nuclear pore complexes (NPCs) is plotted for four molecules (A to D) as a function of time, starting from an arbitrarily chosen initial concentration difference. Indicate which curve corresponds to each of the following molecules. Your answer would be a four-letter string composed of letters A to D only, e.g. ABCD.

( ) A large protein that is being actively transported across the NPC
( ) A small water-soluble molecule
( ) A small protein composed of a few dozen residues
( ) A large protein that is NOT actively transported into or out of the nucleus