In a pattern of letters: X, Y, Z, A, B, ?, D. What is the missing letter?

A. C

In the pattern 5, 10, 15, ?, 25, what number is missing?

A. 20

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

A. 16

In the series X, Y, ?, A, B, C, which character is missing?

A. Z

What character should replace the question mark in the pattern X, Y, Z, A, B, ?

A. C