萨拉注意到他的手有轻微的颤抖。

Sarah ___observed a slight tremor in his hands.

你注意到是谁把这张纸条放在你桌上了吗？

Did you ___observe who left this note on your desk?

研究人员观察到这些动物的行为发生了显著变化。

The researchers ___observed a significant change in the animals' behavior.