A).She can measure m∠2 and m∠3 with a protractor.
B).She can subtract 180° from both sides of m∠1+m∠2=180° and m∠1+m∠3=180°.
C).She can subtract m∠1 from both sides of m∠1+m∠2=180° and m∠1+m∠3=180°.
D).She can subtract m∠1 from both sides of m∠1+m∠2=180°, and subtract m∠3 from both sides of m∠1+m∠3=180°.