Which of these sentences could be a main idea in a paper about teenagers using computers responsibly? 1. It’s healthier for teenagers to be outside than on the computer.
2. Email is a great way for older people to connect with old friends.
3. What you say online can easily spread beyond your control.
4. In the previous century, there were no personal computers or smart phones.

3. What you say online can easily spread beyond your control.  

Explanation:

I'm pretty sure that would be the main idea on a paper about teenagers using computers responsibly. This is probably because they are trying to teach you that what you put out on the internet can never be taken out and that you should be careful with whatever you say.

probably because the owner wants to get rid of it

the longest time is 123 years