请用英文回答如下问题： （1) What are “hidden terminals” and, （2) how do they affect the performance of wireless sensor networks？

How are sentence meaning and utterance meaning related and how do they differ? 句子意义和话语意义有什么联系与区别?

听力原文：How do they feel now?

(12)

A．They are feeling very tired.

B．They think the climbing is rewarding.

C．They are a little bit disappointed.

D．They don't feel anything special.

delicious cakes they are!

A、What

B、How

C、What a

The question of what children learn and how they should learn it is continually being debated and redebated. Nobody dares any longer to defend the old system, the learning of lessons parrot-fashion and the grammar with-a-whip system, which was good enough for our grandparents. The theorists of modem psychology have stepped in to argue that we must understand the needs of children. Children are not just small adults; they are children who must be respected as such.

Well, you may say, this is as it should be, and a good idea. But think further. What happens? "Education" becomes the responsibility not of teachers, but of psychologists. What happens then? Teachers worry too much about the psychological implications of their lessons, and forget about the subjects themselves. If a child dislikes a lesson, the teacher feels that it is his fault, not the child's. So teachers worry whether history is "relevant" to modem young children. And do they dare to recount stories about violent battles? Or will this make the children themselves violent? Can they tell their classes about children of different races, or will this encourage racial hatred? Why teach children to write grammatical sentences? Verbal expression is better. Sums? Arithmetic? No, no; real-life mathematical situations are more understandable.

You see you can go too far. Influenced by educational theorists, who have nothing better to do than write books about their ideas, teachers leave their teacher-training colleges filled with grand, psychological ideas about children and their needs. They make elaborate, sophisticated preparations and try out their "modem methods" on the long-suffering children. Since one "modem method" rapidly replaces another, the poor kids will have had a good bellyful by the time they leave school. Frequently the modem methods are so sophisticated that they fail to be understood by the teachers, let alone the children; even more often, the relaxed discipline, so essential for the "informal" feeling the class must have, prevents all but a handful of children from learning anything.

People dare not defend the old system mainly because under the old system ______.

A．children were made to learn passively

B．children were spoiled

C．children were treated as grown-ups

D．too much grammar was taught to children