Can you attach the entire article because it may be either A or D?
I think it would be “who weren’t able to go on the field trip”
E. An abbreviation for id est, a Latin phrase meaning “that is.” It indicates that an explanation or paraphrase is about to follow: “Many workers expect to put in a forty-hour week — i.e., to work eight hours a day.”
Explanation:
Astronomers have identified more spiral galaxies than ellipticals, but that's simply because the spirals are easier to spot. While spiral galaxies are bright, elliptical galaxies are dim. The existing stars inside an elliptical galaxy tend to be older, giving off more red light than younger stars.
Im confused on your question