6 Choose the correct answer. 10 The boys aren't/weren't/didn't working on their project when the teacher sent them the message.

Краткое пояснение:

Выбираем правильную форму отрицания для Past Continuous, так как действие происходило в прошлом и было прервано другим действием (when the teacher sent them the message).

Пошаговое решение:

1. aren't working — отрицание Present Continuous.
2. weren't working — отрицание Past Continuous.
3. didn't working — неправильная форма (после didn't используется инфинитив без -ing).

Ответ: 10. The boys weren't working on their project when the teacher sent them the message.