Beijing S 1 recruitment

(1) After the switch S 1 is closed, R2 is short-circuited when S2 is closed, and U 1=U=6V.

(2) When the switch S 1 is closed and S2 is open, R 1 and R2 are connected in series.

∫r 1 = R2

∴u 1′=u2= 12u= 12×？ 6V=3V

So choose a.