A study has been conducted investigating whether depression has an impact on memory. The null hypothesis is that the mean score on a cognitive test is the same for elderly people with depression and elderly people without depression. The average score for depressed participants was 78 / 100 The average score for non-depressed participants was 84 / 100 The 95% confidence interval for the observed difference does not include zero. The p value was calculated and is less than 0.01. The researchers should: Select one: O a. Fail to reject the null hypothesis O b. Retain the null hypothesis O c. Accept the null hypothesis O d. Reject the null hypothesis