A farmer investigated pesticide contamination at a small pond with significant run-off (meaning water runs from the nearby fields into the pond). Eight measurements of pesticide concentrations were randomly taken at the pond (assume the pesticide is normally distributed throughout the pond). The reported pesticide concentrations: 15.1, 13.2, 15.3, 14.7, 11.0, 15.2, 16.5, 18.1 g/L. Given that the data distribution is normal, find the t-value for a 90% confidence level.

a	2.306
b	1.860
c	1.895
d	2.365
e	None of the above