Write the correct form: 1) Listen! Someone (to knock) at the door. 2) Don't go into the classroom! The students (to write)

1) Listen! Someone is knocking at the door. (Present Continuous). 2) Don\'t go into the classroom! The students are writing a dictation there. (Present Continuous). 3) They have been discussing it since five o\'clock. (Present Perfect Continuous). 4) — Where is Petrov ? — He is working in the library. (Present Continuous). — Has he been working long? (Present Perfect Continuous). — Yes, he has been working since morning. (Present Perfect Continuous).