The temperature of an ideal gas is raised from 10°C to 20°C while the volume is kept constant. If the pressure at 10°C is P, what is P_f, the pressure at 20°C?

1. P_f < P (4/35) (11%)
2. P_f = P (0/35) (0%)
3. P < P_f < 2P (14/35) (40%)
4. P_f = 2P (13/35) (37%)
5. P_f > 2P (4/35) (11%)