Soprano: C-B-A-A-G (descending)
Bass: A-B-C (ascending), then down to D and up to G.
Can you even add tenor and alto without breaking the rules of harmony? It seems like it is impossible.
So what is the best way to harmonize this? I did try it with 3 voices but ended up with B-D-B on the second beat. This is not correct as a sixth interval is good, but we have a G chord without G and with two thirds. You don't want to end up with B-E-B since you'll end up with a fifth.