Answer:
Option D, has only a single opening
Step-by-step explanation:
A gastrovascular cavity has only one opening. The food goes in and comes out of the same opening there by making it a two way digestive tract.
However, a digestive tube is a one way digestive tract where the food goes in from mouth (one end) and comes out of anus (second end) . It is also called as alimentary canal
Hence, option D is correct