Identify the missing character in the series 1, 4, 9, ?, 25, 36.

A. 16

In the series X, Y, Z, ?, B, C, what letter should replace the question mark?

A. A

In the sequence 3, 6, 9, ?, 15, 18, what number should replace the question mark?

A. 12

In the series: 1, 4, 9, ?, 25, what number should replace the question mark?

A. 16

In a sequence of letters: A, C, E, G, ?, K. What is the missing letter?

C. H