To make a secure fit, rivets larger than the holes are often used. The rivets are cooled in dry ice before put in. Steel has a linear expansion coefficient 12x10-6 /°C. The diameter is 2.5 cm for a hole, and 2.5 + 0.001 cm for a steel rivet at 20°C. The rivet needs to be cooled to _____ °C beforehand.