Sasha selected a random sample of bivariate data, with a sample size of 30, and calculated a slope of 3.6 for the sample slope of a regression model. Sasha constructed a 95 percent confidence interval to estimate the slope. Alex claims he can construct a confidence interval that is narrower by changing the sample size but keeping all other things the same. Which of the following sample sizes will make Alex's claim true?
A. 10
B. 15
C. 20
D. 25
E. 35