An independent random sample is selected from an approximately normal population with unknown standard deviation. Find the degrees of freedom and the critical t-value (t*) for the given sample size and confidence level. 

(a) n = 6, CL = 90% 

(b) n = 21, CL = 98% 

(c) n = 29, CL = 95% 

(d) n = 12, CL = 99%