题目内容：

    Language learning apps are very popular now and offer opportunities to learn vocabulary and practice grammar. But there has been a discussion about just how effective such apps can be.

Among the most popular apps are Duolingo and Busuu. A former research found positive results on the use of them. But it mainly concentrated on studies with learners who had signed up for language courses and these apps worked as an after-class support, so the results were not always reliable. A recent study of 4,095 Busuu users has been carried out to find out if users can actually learn a language with an app.

Busuu provides learning materials for 12 different languages. It offers special models where some contents are ready for use for free while some contents are not. We find that its users are an even mix of men and women. More than half consider themselves to be at the beginner level because there is a reduction in users as their language levels go up. Most people use the app because of personal interest, or because they want to study or live abroad. Female users are likely to use it less often but for longer periods of time than male users.

Feedback in the app is highly thought of. More than 92% of the users say that the app has met their expectations and 86% consider the app as very good or good. In fact, more than 80% of the users surveyed strongly agree that using the app has helped them improve their knowledge of the language they are learning.

Besides, language learning apps also create an environment where mistakes are only known to the users, and this can address the performance nervousness that many learners suffer from when asked to speak a foreign language. So language teachers should encourage their students to use them to do the grammar work, leaving precious class time for more language communication.

1.What's the reason why the results of the former research weren't always reliable?

A.It only considered users of a limited kind.

B.It just focused on positive results.

C.It was only carried out before class.

D.It used out-of-date methods.

2.What can we learn about the app Busuu from the text?

A.It's used by more advanced learners.

B.. All the contents are free to use.

C.Most users consider it satisfying.

D.It's more popular with female learners.

3.The underlined word"address"in the last paragraph can be replaced by_____.

A.cause B.solve

C.increase D.practice

4.What is the main idea of the text?

A.Language learning apps are very popular with most language teachers.

B.Students won't suffer nervousness in learning languages by using Busuu.

C.Language learning apps are effective and satisfactory to language learners.

D.Students should be required to use apps for homework after class.