The text just transitions from the hours spent on internet by teenagers to isolation and obesity. This indicates that the text is trying to sggest that excessive use of the internet is causing isolaiton and obesity.
A – Wrong
The text is not focused on the benefits of Internet. So A is irrelevant.
B – Wrong
The text is meant to focus only on teenagers who use the internet excessively.
C – Wrong
It does not assume parents can enforce stricter controls. It just states that parents must enforce stricter controls. There is a difference between assuming and advising.
D – Correct
TV and computer games are irrelevant as the text is meant to focus only on internet.
E – Wrong
This identifies the flaw which is the causal link made by the text between internet and isolation and obesity.