<span>The muscle at the end of the esophagus is not able to close, which allows stomach acid and digested food to go back up into the esophagus. </span>
<span>The grouping of organisms based on their common descent is called evolutionary classification. Through evolution, one species has changed and become modified until it became what it is today. However, some of today's organisms have the same predecessor, which is evident if you take evolution into consideration. </span>
The first image is the answer.
