capillary|capillaries

科学家用毛细管精确测量液体的上升高度。

Scientists use a ___capillary to measure the precise height of liquid rising.

毛细血管非常细小，只有在显微镜下才能看到。

___Capillaries are so tiny that they can only be seen under a microscope.

毛细血管非常细小，肉眼几乎看不见。

___Capillaries are so tiny that they're almost invisible to the naked eye.