Final answer:
The sentence that reflects the theme of considering how actions affect others is the one expressing the girls' disappointment with the man leaving his net unattended. It showcases the negative impact on wildlife and aligns with the theme of responsibility towards the environment.
Step-by-step explanation:
The sentence from the story "Sam and Julie to the Rescue" that supports the theme that you should consider how your actions affect others is: "The girls were disappointed that the man would leave his net unattended." This sentence demonstrates a realization of the repercussions the man's inattention can have on the environment, affecting the ability of the duck to swim safely. It is a clear example of how an individual's actions, even if not intended to harm, can have negative effects on others. This is aligned with the broader theme of responsibility and consciousness of one's impact on their surroundings.