At the Beaver Construction factory, Benoit works at the nut 
and bolt 
assembly line.在比弗建筑工厂，贝努瓦在螺母和螺栓装配线上工作。
His job description is as follows:他的工作职责如下：
- Stand at one end of a conveyor belt, which brings a line of nuts and bolts.站在传送带的一端，传送带上有一排螺母和螺栓。
- Take each element, either a nut or a bolt, off the conveyor belt.把传送带上的每个零件，不管是螺母还是螺栓，都取下来。
- Put the nuts from the conveyor belt in the red bucket.把传送带上的坚果放进红色的桶里。
- Attach any bolt, taken from the conveyor belt, to a nut from the red bucket and place the assembled part onto the table behind you.从传送带上取下任意一个螺栓，与红色桶里的一个螺母拧在一起，然后把拧好的部件放到你身后的桌子上。
However, things can go wrong for Benoit in two different ways:然而，对于贝努瓦来说，事情可能会以两种不同的方式出错：
- When Benoit takes a bolt from the conveyor belt there may not be a nut in the bucket.当贝努瓦从传送带上取下一颗螺栓时，桶里可能没有螺母。
- There may be no more nuts or bolts on the conveyor belt but there are still nuts in the bucket.传送带上可能已经没有螺母或螺栓了，但桶里还有螺母。
Question: 问题:
Which sequence of nuts and bolts, when processed from left-to-right, will not cause things to go wrong for Benoit?从左至右依次加工哪一组螺母和螺栓，才不会让贝努瓦出错？
A. 
B. 
C. 
D.
Explanation 解释
Answer: 答案:
C.
Explanation: 解释:
We can keep track of the state of the bucket and conveyor belt from left-to-right:我们可以从左到右跟踪料斗和传送带的状态：
- N =
- B =
| Bucket | Conveyor Belt |
| empty | N B N N B N N B B B |
| N | B N N B N N B B B |
| empty | N N B N N B B B |
| N | N B N N B B B |
| N N | B N N B B B |
| N | N N B B B |
| N N | N B B B |
| N N N | B B B |
| N N | B B |
| N | B |
| empty | empty |
Looking at the other answers:看看其他答案：
will go wrong after N B B, since there will be no nut in the bucket when the second B is encountered.在执行了 N 次 B B 操作之后将会出错，因为在遇到第二个 B 时桶里将没有坚果了。
will go wrong after N N B N N B B B B, since there will be no nut in the bucket when the fifth B is encountered: notice there are only 4 N's before this B.在遇到第 5 个 B 时将会出错，因为此时桶里已没有坚果了：请注意，在这个 B 之前只有 4 个 N。
will go wrong after the entire sequence is processed, since there will be two nuts in the bucket, since there are 6 N's and 4 B's.在整个序列处理完毕后会出现错误，因为桶里会有两个螺母，因为有 6 个 N 和 4 个 B。
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