阅读短文，下列问题
Dear Abby，
I'm a helpless mother. My son Tom is 15 years old. I find that the older he grows, the less we talk. I feel very sad and I really need help.
My first problem is about his hobbies. He spends almost all his spare time on computer games. When he gets home, he always turns on the computer and closes the door. I'm getting worriedabout him, especially when his English teacher tells me he always gets low marks in the English exams.
Second, he likes newtechnology，but I don't have enough money and I can't afford all the things he wants.
Recently, I've foundthat he is always alone. I've never seen him talking to his friends on the phone or going out with anyone on weekends. That is the last problem I have.
What should I do to help him? Pleasegive me some advice.
A helpless mother
的介质中，传播速度变为____。