Cells vary in size as well as shape, but all cells are very small.
Bacteria are single-cell microorganisms that are so small they can only be seen with a microscope.
Cells vary in size and shape and can only be seen with a microscope.
Cells vary in size and shape and can only be seen with what?

(a) a microscope
(b) heightened senses
(c) a telescope lens
(d) colors
(e) Solar energy
(f) by indirect light
(g) a good eye
(h) A computer