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A silicon wafer has $100 \mathrm{~nm}$ of oxide on it and is inserted in a furnace at a temperature above $1000^{\circ} \mathrm{C}$ for further oxidation in dry oxygen. The oxidation rate

1. is independent of current oxide thickness and temperature
2. is independent of current oxide thickness but depends on temperature
3. slows down as the oxide grows
4. is zero as the existing oxide prevents further oxidation