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问题 单项选择题

对如下二叉树,进行后序遍历的结果为【 】

A．ABCDEF

B．DBEAFC

C．ABDECF

D．DEBFCA

答案

参考答案：D

解析：二叉树后序遍历的简单描述如下：若二叉树为空，则结束返回。否则(1)后序遍历左子树：(2)后序遍历右子树；(3)访问根结点。也就是说，后序遍历是指在访问根结点、遍历左子树与遍历右子树这三者中，首先遍历左子树，然后遍历右子树，最后访问根结点，并且在遍历左、右子树时，仍然先遍历左子树，然后遍历右子树，最后访问根结点。根据后序遍历的算法，后序遍历的结果为DEBFCA。

单项选择题

How to protect children Web fans from unsuitable material on-line while encouraging them to use the Internet has long been discussed in the US.
For some parents, the Internet can seem like a jungle, filled with danger for their children. But jungles contain wonders as well as dangers and with good guides, some education, and a few precautions, the wilds of the Internet can be safely navigated. "Kids have to be on-line. If we tell our kids they can’t have access to the Internet, we’re cutting them off from their future, " said an expert.
Most kids have started to use search engines. Many of them are great for finding tons of interesting Internet sites, and they can also locate places where you might not want your kids to go. There are search engines designed just for kids. A certain software contains only sites that have been selected as safe. The most popular way to limit access would be to use what is known as a "content screener". But this can’t be wholly reliable, and the best thing parents can do is to talk to their kids and let them know what is OK or not OK to see or do on the Internet. Another way is that mum or dad is nearby when the child is surfing the Internet.
A few other tips
·Don’t put the PC in a child’s room but keep it in an area where mum or dad can keep an eye on things. That also makes the Internet more of a family activity.
·Ask your child what he or she has been doing and about any friends they make on-line.
·Tell your child not to give on-line strangers personal information, especially like address and phone number.
·And tell your children never to talk to anyone they meet on-line over the phone, send them anything, accept anything from them or agree to meet with them unless you go along.

The best way to protect children from improper material is ______.

A. to install (安装) a content screener on the computer
B. to buy some search engines for the children
C. to be nearby when they are surfing the Internet
D. to talk to the children and persuade them to tell right from wrong
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选择题

Quantities of water ________ every day with no people noticing it. [ ]

A. is wasted  

B. is wasting

C. are wasting  

D. are wasted
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