Which number will come next in the following series?
$2, 1, 2, 1, 3, 2, 1, 3, 4, 2, 1, 3, 4, 5, 2, 1, 3, 4, 5, 6, 2, 1, 3, 4, 5, ?$

How many such $4$s are there in the following number series,each of which is immediately followed by $5$ but not immediately preceded by $3$?
$5, 4, 4, 3, 3, 4, 5, 4, 5, 5, 3, 4, 4, 3, 4, 5, 4, 3, 3, 5, 4, 5, 4, 3, 5, 4, 4, 3, 5, 4$

Choose the correct alternative answer

$adb...ac...da...eddeb...dbc...cbda$

Find the missing letters for the given series from the given alternatives:
$abca.....bcaab....,aa.....caa....c$

Find the missing letters for the given series from the given alternatives:
$ab....bbc....c....ab....ab....b$

Choose the correct alternative to complete the series: $a \_ n \_ b \_ \,, \, \_ n c b \_ \,, \, \_ n c b$