12.21 A large company buys thousands of lightbulbs every year. The company is currently considering four brands of lightbulbs to choose from. Before the company decides which lightbulbs to buy, it wants to investigate if the mean lifetimes of the four types of lightbulbs are the same. The company's research department randomly selected a few bulbs of each type and tested them. The following table lists the num- ber of hours (in thousands) that each of the bulbs in each brand lasted before being burned out. Brand I 23 Brand II 19 Brand III Brand IV 23 26 24 23 27 24 19 18 25 21 26 24 26 29 22 20 23 28 23 22 21 24 25 19 27 28 At the 2.5% significance level, test the null hypothesis that the mean lifetime of bulbs for each of these four brands is the same.