【求助】一道GAME中的一个小问卡住了！！！

AngelinWang

in a game, "words" (real or nonsensical) consist of any combination of at least four letters of the English alphabet. Any "sentence" consists of exactly five words and satisfies the following conditions:
The five words are written from left to right on a single line in alphabetical order.

（这五个单词按照字母顺序，从左到有，在一个单独的横行书写。请问这句话是这么理解吗？）
the sentence is started by any word, and each successive word is formed by applying exactly one of three operations to the word immediately to its left: delete one letter, add one letter or replace one letter with another letter.
at most three of the five words begin with the same letter as one another.
except for the leftmost word, each word is formed by a different operation from that which formed the word immediately to its left.

QUESTION #14
the last letter of the alphabet that the first word of a sentence in the word game can begin with is（在这个游戏中，一句话的第一个单词的最后一个字母可以用哪一个开头？请问这句话是这么理解吗？）
a) t
b) w
c) x
d) y
e) z

为什么正确答案是D呢？

烦请大家见谅，我的这个问题可能有点傻，应该是哪里卡住了，诚盼大家解答！

非常非常感谢啊！

simonshen

这题因为说了at most three of the five words begin with the same letter as one another，就是五个word里最多只有三个能用同一字母起头，而且这五个word的第一个字母也得是按字母表顺序排列的。如果第一个word的第一个字母是Z（字母表上最后一个字母），那后面的word只能也以Z开头，那样就5个word都必须以同一字母开头了，这样就与五个word里最多三个能用同一字母开头这个规则矛盾了。所以第一个word最多只能用y开头，然后后面2-5个word的第一个字母可以依次为Y，Y，Z，Z或者Y，Z，Z，Z。
这题可以随便举个例子
yabc, yabd, yabde, zabde, zabd

AngelinWang

simon讲的好清楚啊！万分感谢！！！^_^