A, paper and adviser can assist you to get green card after graduation ASAP
. Or it's better directly transfer to CS than selecting B.
BTW, staying away from inorganic, physical, theoretical or some organic
chemistry. It's painful to find a job after graduation. Analytical and
synthesis are way better, but it's relatively hard to publish paper.
An alternative way is selecting A, and get a master degree in organic or
analytical chemistry. I knew some guy are working in this degree level with
GC obtained from Phd achievements. So called 曲线救国 la , Z turn.
Much better method is just get master degree in CS or chemistry, marrying an
American, or green card holder, or your PhD classmate. Last one is more
common for Chinese, I knew some guys are obtaining GC following their Phd
wives. So there is no shame for that :). Marrying ones from other
counties is also option: so you don't wait too long for GC and also give
left of us an extra quota.