$3.$ Write regular expressions for the following languages:

$($a$)(5$ marks$)$ The set of strings of $0\text{'}$s and $1\text{'}$s whose fth symbol from the right end is $1.$

$($b$)(5$ marks$)$ The set of $0\text{'}$s and $1\text{'}$s with at leastone pair of consecutive $1\text{'}$s$.$

$($c$)(5$ marks$)$ The set of all strings of $0\text{'}$s and $1\text{'}$s not containing $101$ as a substring