An engineer designs an improved light bulb. The previous design had an average lifetime of 1200 hours. The new bulb has a lifetime of 1201 hours, using a sample of 2000 bulbs. Although the difference is quite small, the effect is statistically significant. The explanation is: a. that new designs typically have more variability than standard designs b. that the sample size is very large. c. that the mean of 1200 is large. d. all of the above