HiFly runs a flight school and uses an activity$-$based costing system. Flight hours are often used as an allocation base. Lessons can range from $30$ minutes to $120$ minutes of flight time. HiFly uses various aircraft. Classify the following costs as unit$-$level, batch$-$level, product$-$line, or facility$-$support.

$\backslash$table$[[$Cost$,$Level$],[$a$.$ Aircraft maintenance $($maintenance scheduled is based on flight time$),$Unit$-$level$],[$b$.$ Aircraft hangar rental,Facility$-$support$],[$c$.$ Aircraft depreciation,$],[$d$.$ Aircraft insurance,$],[$e$.$ Pilot certification $($pilots must be certified for each aircraft type they fly$),],[$f$.$ Aircraft safety inspection $($performed after each flight lesson$),$Product$-$line$]$