Which of the following is not required to be displayed as a warning device when a truck carrying flammable liquids that is over 30 feet long is disabled on the roadway for more than 10 minutes?

a. During daytime in clear weather, one red flag placed 100 feet to the rear and one red flag placed 100 feed in advance of the vehicle
b. A lighted flare placed approximately 100 feet from the disabled vehicle in the center of the lane occupied by such vehicle and toward approaching traffic
c. a portable emergency reflector 100 feet in the opposite direction from the disabled vehicle
d. a red reflector placed on the traffic side of the disabled vehicle not less than 10 feet in the rear and in the direction of traffic.