What is the next letter in the series: A, E, I, M, ?

A. O

A, C, E, G, ?

B. H

What is the missing term in the series: 3, 7, 15, 31, ?, 127?

A. 63

Complete the series: a, c, e, g, ?

B. H

Find the missing term in the series: 1, 1, 2, 3, 5, ?, 21.

A. 8