Why Can't You Directly Observe, with the Unaided Eye, Brownian

Why can't you directly observe, with the unaided eye, Brownian motion in easily visible objects such as floating bits of paper?

A) Because bits of paper are so massive [or heavy] that they do not respond noticeably to atomic impacts.
B) Because paper cannot be electrically charged, so it cannot respond to Brownian forces by individual atoms.
C) Because atoms are so small that you can't see them with the unaided eye.
D) Because only living organisms such as bacteria exhibit Brownian motion.
E) Because only individual atoms and molecules exhibit Brownian motion.