You are overtaking a tractor-trailer on the highway and, as you draw alongside, the steering feels suddenly light and unsettled. What is happening?

Correct answer: B. Air turbulence off the large vehicle — hold the wheel firmly and complete the pass rather than lingering alongside

You have driven into the truck's air turbulence. The Handbook lists it as one of a large vehicle's limitations you have to plan around: "Due to various factors such as air pressure and airflow, a large vehicle can create heavy air turbulence. This may affect your ability to control your vehicle when passing a large one." Expect it, hold the wheel firmly, and get the pass done rather than lingering beside the trailer. The same section names its wet-weather twin — "in bad weather, large vehicles are capable of spraying up large amounts of mud, snow and debris, which could land on your windshield and temporarily block your vision." Both are reasons to pass a truck decisively or not at all, rather than sitting alongside it.

Source: MTO Driver's Handbook: Sharing the road with large commercial vehicles — Turbulence and spray