Why is a mobile phone network called a cellular phone network?

The reliability of a hard-disk drive is typically described in terms of a quantity called mean time between failures (mtbf). although this quantity is called a “time,” the mtbf actually is measured in drive-hours per failure. if a system contains 1,000 disk drives, each of which has a 750,000-hour mtbf, which of the following best describes how often a drive failure will occur in that disk farm: once per thousand years, once per century, once per decade, once per year, once per month, once per week, once per day, once per hour, once per minute, or once per second?