Which of the following statements are true? There may be more than one correct statement; check all that are true. a) Both the pooled-variance t and Welch t assume the population means are equal. (b) The pooled-variance t works much better than Welch when the population variances are very different. (c) The pooled-variance t and Welch's t have the same degrees of freedom, but different test statistics. d) If we are in a two-sample inference situation where the population variances are very different and the sample sizes are very different, it is better to use Welch than the pooled-variance t