Identify the missing letter in the series: b, d, f, h, ?

D. I

Complete the series: a, e, i, o, ?

A. U

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

A. 32

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

C. 80

What number should replace the question mark in the series: 1, 1, 2, 3, 5, ?

A. 7