Here are instructions for making a sailor necklace using blue striped beads and solid red beads:以下是使用蓝色条纹珠子和纯红色珠子制作水手项链的说明：
- Every sailor necklace starts by placing one blue striped bead and one red bead on a string.每条水手项链都是先在一根线上串上一颗蓝色条纹珠子和一颗红色珠子开始的。
- The sailor necklace can then be made longer by either:水手项链随后可以通过以下任一方式变长：
- Adding two blue striped beads to the right end of the string;在串珠的右端加上两颗蓝色条纹珠子；
- Or adding a red bead to each end of the string.或者在绳子的两端各加上一颗红色的珠子。
These two actions can be done many times in any order to build longer and longer necklaces.这两个动作可以以任意顺序重复多次，从而制作出越来越长的项链。
Question:问题:
Which necklace below is NOT a sailor necklace? 下面哪条项链不是水手项链？
A.
B. 
C. 
D. 
Explanation 解释
The correct answer is D. . There are many ways you could have solved this task.解决这个任务的方法有很多。
We will name action one "B" and action two "W".我们将把第一个动作命名为“B”，把第二个动作命名为“W”。
Here are three possibilities:以下是三种可能性：
- You could have constructed each necklace by first locating the two starting beads and then perform a series of B and W actions. Necklace can be made starting with the second and third beads, and then performing actions W-B-B. Necklace can be made starting with the third and fourth beads, and then performing the actions W-W-B. Necklace can be made starting with the second and third beads, and then performing the actions B-W-B. However, if you look at necklace , the starting beads must be the second and third beads. You can perform action W once, but then there are no actions that will construct the rest of the necklace.你可以通过先确定两个起始珠子，然后依次执行一系列的 B 和 W 操作来制作每条项链。项链 可以从第二和第三颗珠子开始，然后执行 W-B-B 操作。项链 可以从第三和第四颗珠子开始，然后执行 W-W-B 操作。项链 可以从第二和第三颗珠子开始，然后执行 B-W-B 操作。然而，如果你看项链 ，起始珠子必须是第二和第三颗珠子。你可以执行一次 W 操作，但之后没有操作可以完成项链的其余部分。
- This approach would not work well if the necklace was very long and had many possible starting beads. In this case, you could instead try a deconstructive approach where you repeatedly remove beads by reversing the B and W actions until you only have two starting beads left.如果项链很长且有多种可能的起始珠子，这种方法就不太管用了。在这种情况下，您可以尝试一种反向构建的方法，即反复通过反转黑白操作来移除珠子，直到只剩下两个起始珠子为止。
- A third strategy is to notice something about the parity of the beads. According to the sailor necklace instructions, there will always be an odd number of solid red beads and an odd number of striped blue beads. Can you see why? Necklace has an even number of both types of beads and therefore cannot be a sailor necklace. 第三种策略是留意珠子的奇偶性。根据水手项链的制作说明，实心红珠子的数量和条纹蓝珠子的数量总是奇数。你能明白其中的原因吗？项链 两种颜色的珠子数量都是偶数，所以它不可能是水手项链。
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