Eva wants to estimate the mean number of plants per square meter in a local forest. She marks off six different one-square-meter plots in different parts of the forest, and finds the following number of plants in each: 11, 15, 25, 17, 36, 22. Within which scores can Eva be 95% sure that the population mean number of plants per square meter lies?

a. 9.5 to 32.7

b. 11.7 to 30.3

c. 13.9 to 28.1

d. 15.2 to 26.7