If you have never seen a power plant, you might find it hard to imagine how enormously complex the equipment is or how much heat is generated by the boilers or how much coal it takes to fuel the furnaces for just one day. During the course of a day, the boilers at one of our power plants, Morgantown, for example, can turn 24 million gallons of water into steam. That generating plant alone uses 9,900 tons of coal in its furnaces in just one day. Potomac Electric Power Company (PEPCO) uses so much coal that we have purchased two 80-car unit trains to facilitate the delivery process. And Morgantown alone can produce over 25 million kilowatthours of electricity in a single day’s operation. Electricity is produced by spinning large magnets inside a coil of wire within the generator. The faster we spin the magnets, the higher will be the voltage of the electricity produced. Electricity leaves a PEPCO generator at between 13,800 and 24,000 volts. The next step in the process occurs when electricity passes through a transformer where the voltage is stepped up to continue on its journey. A transmission wire is like a small diameter pipe. Stepping up the voltage is like increasing water pressure, thus speeding the flow of energy through the system.

一直齿齿条齿距为8πm，则齿条模数为（）。

A.4mm

B.2mm

C.8mm

D.6mm