Answer:
An avalanche happened when a huge area of snow melted.
Explanation:

The answer is “D. They are easily distracted.”
This is because active listeners are fully concentrated people.
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Answer:
Depending on what you mean, it is probably the body paragraph.
Explanation:
Answer:
C. After dolphins have been underwater for a long period.
Explanation:
A sentence fragement is an incomplete sentence/thought. In all the rest of the answers, there is a concrete meaning behind what is being saif. However, in C, the write doesn't tell you what happens after a dolphin has been underwater for a long period. Thus, it's an incomplete thought and a sentence fragment.
A similar pair would be answer C.