Which of the following statements correctly describes the relationship between two adjacent angles whose measures add up to 180∘?
a) If two adjacent angles are equal, their non-common arms form a straight line.
b) If two adjacent angles sum to 90∘, their non-common arms form a straight line.
c) If two adjacent angles sum to 180∘, their non-common arms form a straight line.
d) If two adjacent angles sum to 180∘, their common arms form a straight line.