题型分类：简化句子题

原文分析：简化题考察找主干和逻辑的能力，原文逻辑 but，主要内容是 these colors would not be very visible if the animal was sitting in an area with a yellowish or greenish background。

选项分析：

D选项 Although 和原文 but 逻辑一致，且包含了主要信息，选项中 poor signal 与原文 not be very visible 属于同义改写。

A选项中 if the background is brightly lit 与原文句子 sitting in an area with a yellowish or greenish background 不一致。

B选项 unless 逻辑与原文不一致。

C选项 be bright enough to be seen 与原文 not be very visible 相反。