Find the missing letter in the series: M, O, Q, S, ?

A. T

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

A. 80

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

B. 13

Identify the missing letter in the series: a, e, i, o, ?

A. U

Complete the series: z, x, v, t, ?

A. R