问题补充：
The last Thursday of November, as a day of thanksgiving in western countries, is an important festival.
A. observing
B. being observed
C. to be observed
D. observed
答案：
A. observing
B. being observed
C. to be observed
(答案→)D. observed
解析：考察非谓语动词。本题中句中主语the last Thursday of November与动词observe(庆祝)，构成被动关系，故使用过去分词短语observed as a day of thanksgiving in western countries作为定语来修饰the last Thursday of November。句意：十一月的最后一个星期四在西方国家被作为感恩节来庆祝，这是一个重要的节日。B项中是现在分词的被动形式，表示正在进行。C项不定式表示未来，均不符合。故D正确。
