New Language App. Experiment

Babbel, a language learning app wants to make a large$-$scale update to their learning system. They want to test if the new system is a more effective way for people to learn a new language.

They get $120$ participants and let each of them choose if they want to use the original app or the new updated version. The participants also choose one of five available languages to learn. $70$ participants chose to use the original app and $50$ chose to use the new app.

After $20$ weeks, all participants took a language assessment test. Then they compared the test scores to see if the new app users scored higher on the assessment test.

A$.$ What is the explanatory variable for this study?

B$.$ What is the response variable for this study i

C$.$ List two things they did correctly when setting up this experiment.

D$.$ List two things they did incorrectly in this experiment. In other words, what requirements of a good experiment are they missing?