Assignment task:
A brush fire rages out of control and is headed for a residential community. In order to stop the spread of the blaze, firefighters spray flame retardant chemicals on Richie's house and yard. The fire is stopped about a mile from Richie's property, but the chemicals have killed Richie's prized Bonzai plants. In Richie's suit against the fire department for damaging his property, the fire department should raise which defense for their actions?