What should Stacy do to explain the relationship between an exterior angle and the interior angles in a triangle? A. Stacy should first show that m∠1+m∠3=m∠2+m∠4. Then she should subtract to show that m∠1=m∠2 and m∠3=m∠4.

B. Stacy should first show that m∠4=m∠3. Then, since m∠1+m∠2+m∠3=180∘, she should substitute to show m∠1+m∠2+m∠4=180∘.

C. Stacy should first show that m∠4+m∠2=m∠1+m∠2+m∠3. Then she should subtract to show that m∠4=m∠1+m∠3.

D. Stacy should first show that m∠4=m∠2. Then, since m∠1+m∠2+m∠3=180∘, she should substitute to show m∠1+m∠4+m∠3=180∘.