Assume that the expected safe life of a heating element for a particular type of furnace is 6,000 hours with a standard deviation of 300 hours, and that the expected safe life conforms to a normal distribution. If the maintenance schedule requires that the element be replaced after 6,000 hours, which one of the following represents the chance that the heating element will become unsafe before being replaced? Select one: O a. 50% because 50% of the normal distribution is below the 6,000 hour mean O b. 60% because there is a 10% tolerance allowance O c. 40% because there is a 10% tolerance allowance O d. 30% because the effect of the standard deviation applied to the 6,000 hour mean