Identify the missing letter in the series: Z, X, V, T, ?

A. R

Identify the missing letter in the series: z, x, v, t, ?

C. Q

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

D. 80

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

A. U

Identify the missing number in the series: 2, 5, 10, 17, ?

B. 25