$```$

#$[$derive $($Debug$)]$

enum Status $\{$

Online $($String$),$

Offline,

$\}$

fn main$()\{$

let user $=$ Status::Online$($String::from$("$Alice$"))$;

if let Status::Online $($name$)=$ user $\{$

println!$("$User $\{\}$ is online.", name$)$;

$\}$ else $\{$

println!$("$User is offline."$)$;

$\}$

$\}$

$```$

Q$9\mathrm{.}1$

$1$ Point

What will the output be$?$

User Alice is online.

User is offline.

Compile Error: Cannot match on borrowed value.

Runtime Error: Enum value was moved.

$1$ Point

Which of the following best describes a Rust trait?

A way to define abstract classes.

A way to specify shared behavior across types.

A mechanism for implementing polymorphism by inheritance.

A feature used to define type aliases.