The U$.$S$.$ Army purchased a large number of Bell Jet Ranger helicopters in $1963.$ Many of the helicopters were used in tours of duty in Vietnam and were returned to the United States after the war. Because of damages sustained during the war, many of the helicopters' engines were rebuilt. In $1974,$ one of the helicopters with a rebuilt engine crashed because of engine failure. The families of the two servicemen killed in the accident wish to recover from Bell under Section $402$A$.$ What defense could Bell probably successfully allege?

Misuse of the product

Product modification

Assumption of risk

Negligence