Atoms of Element a and Atoms of Element B React

Atoms of element A and atoms of element B react to form a compound. In the reaction, the radius of each atom of element A is decreased. Which of the following explains this decrease in atomic radius in the reaction?

A) The atoms of element A lose electrons to atoms of element B.
B) The atoms of element A gain neutrons from atoms of element B.
C) Nuclear particles are converted into energy in atoms of element A.
D) Protons become more densely packed in the nuclei of element A atoms.