—Did you receive the letter the next day?

—No. Somehow, it was three weeks ________ I received it.

A．since

B．before

C．that

D．when

视觉上对彩色的感觉有三个特征，反映颜色种类的特征叫 (36) ，反映颜色深浅程度的叫 (37) ，二者有时通称为 (38) ，另外还有一个特征叫 (39) 。

(1)

A．色调
B．纯度
C．反差
D．色差