For each of the following, identify which kind of variation is at work - random or assignable - and explain your reasoning.
a. During the noon hour, there is an electrical power disruption and nuts machine-tightened during the five minutes that emergency power is on have 15% less torque than normal.
b. Although they are all labeled "9-D," the shoes on a retailer's shelf are not exactly the same size. Any given shoe is just as likely to be slightly larger than 9-D as it is to be slightly smaller.
c. The number of cars leaving downtown Cleveland during a Friday rush hour is greater than during other days of the week.
d. In a production run of curtain rods, the lengths are normally distributed, with a mean of 24.135 inches and a standard deviation of 0.142 inches.