The events in the first paragraph contribute to the reader's understanding of Cassandra's actions is Cassandra realized that her mother was taking too long to pick up Cam, which led to her decision to go help.Thus the correct answer is D.
Any thing which stops you to go ahead and enjoy the moment or achieve something is refers as obstacles.This obstacle can be both internal and external.
In this excerpt is given that Cassandra along with mom and dog visit to Washington square park where the dog joined the doggy daycare group and she and her mother enjoys the performance.
This time her mom got late while picking the dog from the daycare group which creates panic for Cassandra as she never get late.So this is the obstacle which restrict her to enjoy the time which she do always and go the place where her mom picks the dog to help her.
Therefore, option D Cassandra realised that her mother was taking too long to pick up Cam, is the correct answer.
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