Select the letter-pair that can replace the question mark (?) in the following series.

AR, CU, EX, GA, ?

A series is given with one term missing. Select the correct alternative from the given ones that will complete the series.

PON, LKJ, GFE, ?

2, 10, 30, 68, ?

A series is given with one term missing. Select the correct alternative from the given ones that will complete the series.

Narendra Modi, Manmohan Singh, Atal Bihari Vajpayee, ?

Select the option that will fill in the blank and complete the given series.
CDZ, DGE, EJJ, FMO, GPT___

A series is given with one term missing. Choose the correct option from the given ones that will complete the series.

50 : 70 ::..?.... 90

Which of the following numbers will replace the question mark (?) in the given series?
 17, 20, 60, 64, 256, 261, 1305, ?

A series is given with one term missing. Select the correct alternative from the given ones that will complete the series.

5       10      15        20      25 ____?