An expression that represents a single element?

a police officer and a ambulance

it's filtration, but this is an assumption so my apologies if i got this

under a microscope, cancer cells may look very different from normal cells. they often have different sizes and some may be larger than normal while others are smaller. cancer cells are often abnormally shaped and the control centre of the cell (the nucleus) may have an abnormal appearance.

i think? ?