A regular expression is ambiguous when there exists a string that can be constructed in two different ways from the regular expression. Which of the following regular expressions are ambiguous? Explain and provide examples.

$($a$)$ a$(($ab$)$cd $)*\backslash$cup a$($ababcb$)$a$$

$($b$)$ aab$($ab$)\backslash$cup ab$\backslash$cup a$$bba$$

$($c$)$ aaba$\backslash$cup aaaba$\backslash$cup aabba$\backslash$cup a