A:上北下南,左西右东
B:上北下南,左东右西
C:上南下北,左西右东
D:上南下北,左东右西
I can't enter the room because the k____ to the door is lost.
①Cl2+2KI═I2+2KCl ②2FeCl3+2HI═I2+2FeCl2+2HCl
③2FeCl2+Cl2═2FeCl3④I2+SO2+2H2O═2HI+H2SO4.