Put the verb in brackets into the Present Perfect Continuous form. Turn the TV set off. The children (watch) it since morning.

The correct form of the verb in Present Perfect Continuous is "have been watching" because the subject is plural ("children"). So the correct sentence is: Turn the TV set off. The children have been watching it since morning. Explanation in Russian: Необходимо поставить глагол в скобках в форму Present Perfect Continuous. Present Perfect Continuous используется для описания действий, которые начались в прошлом и продолжаются до настоящего момента. Структура Present Perfect Continuous: `have/has + been + verb-ing` В данном случае, так как подлежащее во множественном числе ("children"), мы используем "have". Правильный ответ: have been watching.