18. The shape of a t-distribution with degree of freedom of 299 is

a. Approximately positively skewed.

b. Approximately negatively skewed.

c. Approximately normal.

d. Unknown.

19. For a t test with 40 degrees of freedom, the critical value needed to reject the null hypothesis for a two-tailed test with a = .01 will be:

a. 1.68

b. 2.02

c. 2.71

d. 2.58

20. When a one-sample t-test is conducted, we use instead of for the standard error because:

a. is more accurate than .

b. s is unknown.

c. The t distribution has a different mean than the z distribution.

d. All of the above.