Birds Are Most Closely Related to Reptiles Leading the Authors

Birds are most closely related to reptiles leading the authors to conclude which of the following?

A) That endothermy arose once before the split of mammals and reptiles
B) That endothermy arose once and was then lost in reptiles
C) That birds are actually more closely related to mammals than reptiles
D) That endothermy is not a good way to measure the relationship between species
E) That endothermy arose twice after the split of mammals and reptiles