It’s not easy being a teenager（13至19岁青少年）—nor is it easy being the parent of a teenager. You can make your child feel angry, hurt, or misunderstood by what you say without realizing it yourself. It is important to give your child the space he needs to grow while gently letting him know that you you’ll still be there for him when he needs you.
Expect a lot from your child, just not everything. Except for health and safety problems, such as drug use or careless driving; consider everything else open to discussion. If your child is unwilling to discuss something, don’t insist he tell you what’s on his mind. The more you insist, the more likely that he’ll clam up. Instead , let him attempt to solve（解决）things by himself. At the same time, remind him that you’re always there for him should he seek advice or help. Show respect for your teenager’s privacy （隐私）. Never read him his mail or listen in on personal conversions.
Teach your teenager that the family phone is for the whole family. If your child talks on the family’s telephone for too long, tell him he can talk for15 minutes, but then he must stay off the phone for at least an equal period of time. This not only frees up the line so that other family members can make and receive calls, but teaches your teenager moderation（节制）. Or if you are open to the idea, allow your teenager his own phone that he pays for with his own pocket money or a part –time job
68. The main purpose of the text is to tell parents _____________.
A. how to get along with a teenager
B. how to respect a teenager
C. how to understand a teenager
D. how to help a teenager grow up
69. What does the phrase “clam up” in Paragraph 2 probably mean?
A. become excited B. show respect
C. refuse to talk D. seek help
70. What should parents do in raising a teenager according to the text?
A. Nor allow him to learn driving or take drugs
B. Give him advice only when necessary
C. Let him have his own telephone
D. Not talk about personal things with him
，灯泡正常发光，说明灯泡电压为额定电压，那么变压器输入电压
，变压器副线圈为两个灯泡并联，灯泡电压等于副线圈输出电压即
，原副线圈匝数比为
，选项A错。电压表示数为原线圈输入电压即
，电流表测量副线圈干路电流所以读数为
，选项C错。根据乙图可知输入交流电的频率
，经过变压器后交流电频率不变，选项B对。