Final answer:
The main hurdles for users of the free version of the Duolingo app are the absence of enhanced features and advertisements.
Step-by-step explanation:
The main hurdles for users of the free version of the Duolingo app, as mentioned in the passage, include:
- The absence of enhanced features: The free version of Duolingo does not include new quizzes and the ability to save lessons for offline use. These features are only available in the premium version.
- Advertisements: Users of the free version have to endure advertisements. These ads can disrupt the learning experience.
Therefore, the accurate option that identifies the main hurdles for users of the free version of the Duolingo app is D) Both A and B, as it includes both the absence of enhanced features and advertisements as challenges faced by free users.