Directions: There are two passages in this section. Each passage is followed by some questions or unfinished statements. For each of them there are four choices marked A), B), C) and D). You should decide on the best choice. Passage Two Teens are masters at keeping themselves occupied in the hours after school until way past bedtime. When they're not doing their homework (and when they are) they're online and on their phones, texting, sharing, you name it. Of course before everyone had a Facebook or Instagram account teens kept themselves busy, too, but they were more likely to do their chatting on the phone, or in person when hanging out. It may have looked like a lot of aimless hanging around, but what they were doing was experimenting, trying out skills, and succeeding and failing in tons of tiny real-time (实时的) interactions that kids today are missing out on. Nonetheless, modern teens are learning to do most of their communication while looking at a screen, not another person. Certainly speaking indirectly creates a barrier (障碍) to clear communication, but that's not all. Learning how to make friends is a major part of growing up, and friendship requires a certain amount of risk-taking. This is true for making a new friend, but it's also true for maintaining friendships. When there are problems that need to be faced, it takes courage to be honest about your feelings and then hear what the other person has to say. Learning to effectively cross these bridges is part of what makes friendship fun and exciting, and also scary. But when friendship is conducted online and through texts, kids are doing this when many of the most personal -- and sometimes frightening -- aspects of communication are removed. It's easier to keep your guard up when you're texting, so less is at risk. You aren't hearing or seeing the effect that your words are having on the other person. Because the conversation isn't happening in real time, each party can take more time to consider a response. No wonder kids say calling someone on the phone is "too intense" -- it requires more direct communication, and if you aren't used to that it may well feel scary. If kids aren't getting enough practice relating to people and getting their needs met in person and in real time, many of them will grow up to be adults who are anxious about the primary means of communication for humans -- talking. 30. The author expresses his concern at the end of the passage that ______.