2red and 2 blue overlapping balls in the center are surrounded by a green, fuzzy, circular cloud with a white line running through it. 2 green balls sit on the white line, and a line leading a bracket around the balls is labeled a. a line leading to a bracket overlapping the white line is labeled b.
which is a characteristic of the part of the atom marked "a”?

it contains most of the mass.
it is negatively charged.
it is mostly empty space.
it is composed of electrons