In the series X, Y, Z, A, ?, C, D, what character should fill the gap?

A. B

In the series 2, 4, 6, ?, 10, what is the missing number?

A. 8

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

B. H

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

A. 16

In the series X, Y, Z, ?, B, C, what character should fill the gap?

A. A