What letter comes next in the series: z, x, v, t, ?

A. S

Complete the series: m, n, o, p, ?

A. Q

What number should replace the question mark in the series: 1, 3, 6, 10, ?, 21?

B. 15

Complete the series: b, d, f, h, ?

D. I

Identify the missing number in the series: 12, 24, 48, 96, ?

B. 192