A random sample of 109 light bulbs had a mean life of x=558 hours with a standard deviation of o=32 hours. Construct a 90% confidence interval for the mean life, u, of all light bulbs of this type.
a. 553 hr < u< 563 hr
b. 552 hr < u< 564 hr
c.550 hr < u< 566 hr
d. 551 hr < u< 565 hr