red is a symbol of warmth it can sometimes make people feel uneasy or even frightened. A

问题补充：

red is a symbol of warmth, it can sometimes make people feel uneasy or even frightened.

A. As

B. Since

C. If

D. While

答案：

A. As

B. Since

C. If

(答案→)D. While

解析：考查连词。句子前后表让步关系，排除B,C；as引导让步状语从句时，句子要进行倒装，而原句是正常语序，排除A；所以选D，While虽然，尽管，表让步。