Find the missing number in the series: 5, 10, 20, 40, ?

C. 80

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

A. O

What letter comes next in the series: a, e, i, o, ?

A. U

What is the missing number in the series: 2, 4, 8, 16, ?, 64?

B. 32

Find the missing number in the series: 5, 10, 20, 40, ?, 160.

B. 80