Read the dialogue and choose the correct answer.

Zoey: Hi, Cindy! What are you doing here?

Cindy: Hello, Zoey! (1) I'm currently taking a part-time job here during the summer holiday.

Zoey: Good! How many hours do you work in a week?

Cindy: Because it is not a full-time job, I work 20 hours per week. (2) I always work on weekdays. How about you? What are you doing in this summer holiday?

Cindy: Impressive! (3) I teach English at the Star Learning Center. I work there every morning. Actually, not only English. (4) Sometimes, I also teach math.

Zoey: Oh, really? Good job, Cindy! Do you enjoy teaching?

Cindy: I do. Sometimes it is fun, but sometimes it is tiring.

Zoey: That must be fun. Keep doing great!

Which sentence is grammatically incorrect?