In the addition method, what makes one of the two unknowns disappear when the rules are added together?

1. Adding the rules adds the matching terms of each unknown. 2. Two terms in the same unknown add to zero only when their coefficients are opposites, such as 4 and minus 4. 3. Once that unknown is gone, one rule in one unknown is left, which can be solved directly. 4. Option D would double the term instead of removing it, which is the usual mix-up here. 5. So opposite coefficients are what make the unknown vanish, option A. _Source: OpenStax College Algebra (CC BY 4.0), Ch 7 "Systems of Equations and Inequalities", section 7.1 Systems of Linear Equations: Two Variables_