Answer-As soon as the TV set (A) / was installed, the quiet (B) / stillness that had existed in (C) / my au...

The phrase had vanished is in the past perfect tense, which indicates an action that occurred before another action in the past. In the sentence, had vanished suggests that the vanishing of the quiet stillness happened before the TV set was installed. However, the sentences context implies that the installation of the TV set caused the quiet stillness to disappear immediately, which doesnt align with the sequence of events suggested by the past perfect tense. Instead, using the simple past tense vanished or another appropriate tense would better convey the intended meaning As soon as the TV set was installed, the quiet stillness that existed in my aunts house vanished. This change ensures that the cause TV set installation and effect disappearance of quiet stillness are presented in a more straightforward and chronological manner, in accordance with tense rules.