阅读下列四篇短文，从每小题后所给的A,B,C或D四个选项中，选出最佳选项。
Below is a selection from a popular science book.
If blood is red，why are veins(静脉) blue?
Actually，veins are not blue at all.They are more of a clear，yellowish colour.Although blood looks red when it's outside the body，when it's sitting in a vein near the surface of the skin，it's more of a dark reddish purple colour.At the right depth，these bloodfilled veins reflect less red light than the surrounding skin，making them look blue by comparison.
