Find out the missing letters for the given series from the given alternatives.
$adb.....ac.....da....cddcb.....dbc....cbda$

Find the missing letters for the given series from the given alternatives:
$aaa....bb....aab.....baaa....bb$

Fill in the blank space for the given series with the correct alternative.
$34, 18, 10, 6, 4, (...)$

The following questions are based on the five three-digit numbers given below:
$972, 526, 487, 359, 251$
If $1$ is subtracted from the last digit of each of the above numbers,the sum of the digits of how many of them are prime numbers?

Choose the correct alternative to complete the series: $a . b e . . c . . a b b . . . b c a . . . .$

What should come next in the following letter series?
$nsi, org, pqe, qpc, ?$