Question from the Angular test

@Component({ ... })
export class AppComponent {
  constructor(private userService: UserService){
     userService.getMessages(). subscribe(messages => ...).unsubscribe();
  }
}

@Component({ ... })
export class AppComponent{
  constructor(private userService: UserService){
     const subscription = userService.getMessages().subscribe(messages => ...);
     subscription.close()
  }
}

@Component({ ... })
export class AppComponent implements OnDestroy{
  this.subscription: Subscription<any>;

  constructor(private userService: UserService){
     this.subscription = userService.getMessages().subscribe(messages => ...);
  }

  ngOnDestroy(){
     this.subscription.unsubscribe()
  }
}

@Component({ ... })
export class AppComponent implements OnDestroy{
  this.subscription: Observable<any>;

  constructor(private userService: UserService){
     this.subscription= userService.getMessages()
         this.subscription.subscribe(messages => ...);
  }

  ngOnDestroy(){
     this.subscription.unsubscribe()
  }
}