Scala quiz. Select all correct ways to extend following traits

Select all correct ways to extend following traits.

trait QuizfulOne {
  var value: String
}

trait QuizfulTwo {
  def value: String
}

class QuizfulOneImpl(var value: String) extends QuizfulOne
class QuizfulTwoImpl(var value: String) extends QuizfulTwo

class QuizfulOneImpl(val value: String) extends QuizfulOne
class QuizfulTwoImpl(var value: String) extends QuizfulTwo

class QuizfulOneImpl(var value: String) extends QuizfulOne
class QuizfulTwoImpl(val value: String) extends QuizfulTwo

class QuizfulOneImpl(val value: String) extends QuizfulOne
class QuizfulTwoImpl(val value: String) extends QuizfulTwo

Explanation

A def can be implemented by either of a def, a val, a lazy val or an object. So it's the most abstract form of defining a member. Since traits are usually abstract interfaces, saying you want a val is saying how the implementation should do. If you ask for a val(the same is for a var), an implementing class cannot use a def.

Theory

It's a bad practice to declare abstract vars in abstract classes or traits. Do not do this:
```
trait Foo {
  var value: String
}
```
Instead, prefer to declare abstract things as def:
```
trait Foo {
  def value: String
}

// can then be overridden as anything
class Bar(val value: String) extends Foo
```
The reason has to do with the imposed restriction - a var can only be overridden with a var. The way to allow freedom to choose on inheritance is to use def for abstract members. And why would you impose the restriction to use a var on those that inherit from your interface. def is generic so use it instead.
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