Is a cognitive resource that involves the ability to perform tasks such as deciding whether pairs of two-digit or two-letter strings are the same or different or determining the time required to step on the brakes when the car directly ahead stops.

Perpetual Speed    

Explanation:

               Perpetual Speed is also known as processing speed. It is the speed or the time taken to make mental calculations and mental tasks. It is one of the most important element in the cognitive processes and is therefore the most   important skill in reasoning, learning and experiencing certain things.

             It is the speed with which an individual can understand, perceive and react to the information and data they receive, whether it be auditory (language), or visual (letters and numbers),or movement.

             In other words, processing speed or perpetual speed is the time taken between receiving and reacting to a stimulus.

            The higher the processing speed, the more efficient you are in doing mental tasks.

Thus perpetual speed is the cognitive resource to estimate or calculate the time taken to apply brakes when a car at front of us stops

Hence the answer is ---

Perpetual Speed    

answer; the intervening variable;