I arrived home at midnight, _______ my parents had been waiting for nearly three hours.

A．at that time

B．by that time

C．at which time

D．by which time

油脂的酸败是指油脂和含油脂类的食品在储存过程中通过微生物、酶、空气中的氧气、水分的作用，油脂发生分解、氧化而产生醛、酮及低分子有机酸，使油脂产生哈喇味，从而降低其食用价值。( )