According to a report done by S J Power, the mean lifetime of the light bulbs it manufactures is 44 months. A researcher for a consumer advocate group tests this by selecting 23 bulbs at random. For the bulbs in the sample, the mean lifetime is 39 months. It is known that the population standard deviation of the lifetimes is 9 months. Assume that the population is normally distributed. Can we conclude, at the 0.05 level of significance, that the population mean lifetime, u , of light bulbs made by this manufacturer differs from 44 months?