Which of the following is a sign that a tire is unsafe for highway use?

Federal regulations require steering axle tires to have a minimum tread depth of 4/32 of an inch. Other tires must have at least 2/32 inch. A steering tire with only 2/32 inch tread is well below the legal minimum and is unsafe. Steering tires receive more wear and bear critical responsibility for vehicle control, especially during braking and in wet conditions. Worn steering tires increase stopping distance, reduce wet-weather grip, and dramatically raise the risk of a blowout. A 4/32 inch front tire is at the legal minimum and acceptable. Regroovable tires are permitted on non-steering axles. Mixing two radials is normal and safe; the concern is mixing radial and bias-ply tires on the same axle, which is prohibited.