Complete the series: B, D, F, H, ?

A. J

What number should replace the question mark in the series: 2, 6, 12, 20, ?

B. 42

What is the missing term in the series: 1, 4, 9, 16, ?, 36

B. 25

What is the next letter in the series: A, C, E, G, ?

A. I

Complete the series: a, c, e, g, ?

B. H