Which of the following is false regarding skid marks evidence?

a. all accident scenes have skid mark evidence
b. a skid mark is caused by extreme friction between the rubber tires and the road surface
c. by using a nomograph the investigating officer can estimate the minimum speed that a vehicle was traveling when the wheels locked.
d. Once two elements are known (such as the average length of skid marks and the drag factor) a third unknown (the min speed of a vehicle) can be determined by using a speed nomograph.
e. Since all of the above are true, none responds to the questions