Only when the final examination was coming ________ he had quite a lot to do. [ ]

A. he realized  

B. he had realized

C. did he realize  

D. had he realized

悬浮物在水中的状态，受颗粒本身质量的影响较大。在动水中，由于水的紊流作用，常呈悬浮状态。水静置时，密度较大的颗粒在重力作用下容易自然下沉；密度较小的颗粒，可上浮水面。（）