In which reaction is a small molecule formed from the atoms removed from a single reactant molecule?
addition
condensation
elimination
substitution

ELIMINATION is a reaction where a small molecule from from the atoms is removed from a single reactant molecule. 

addition - reaction where two reactants combine to form a single product; usually occurs in unsaturated compounds

condensation - two molecules combine with the release of a small molecule; aka addition-elimination

substitution - one atom or group of atoms in a compound is replaced by a different atom or group ; usually occurs in saturated compounds

ELIMINATION is a reaction where a small molecule from from the atoms is removed from a single reactant molecule. 

addition - reaction where two reactants combine to form a single product; usually occurs in unsaturated compounds

condensation - two molecules combine with the release of a small molecule; aka addition-elimination

substitution - one atom or group of atoms in a compound is replaced by a different atom or group ; usually occurs in saturated compounds

Change in speed or direction : )

A.(nh4)(2co3)is the correct answer