Which of the following are correct?

$0101$ is a string over the alphabet $=\{0,1,2\}.$

abab is a string over the alphabet $=\{0,1,2\}.$

$\{\{\}\}\{\}\{\{\}\}$ is a strixg over the alphabet $=\{\{\{\}\},\{\}\}.$

Both $($a$)$ and $($c$)$