A 1999 study in the journal Cement and Concrete Research measured the strength of various steel-fibre reinforced concrete samples. One response variable was the number of blows the test samples could withstand before failure. The article states that, for a sample of size n = 30: 95% confidence interval about the mean is 118-166 blows. What does this confidence interval mean?

a. The difference between the population mean number of blows to failure is likely to be between 118 and 166 blows.
b. About 95% of the individual test samples required between 118 and 166 blows to failure.
c. The mean number of blows to failure in the sample is likely to be between 118 and 166 blows.
d. The population mean number of blows to failure is likely to be between 118 and 166 blows.