A young adult is doing a back flip on a trampoline near a lake.
A child jumps into the lake.
The subject is either a young adult or a child, it cannot be both. A young adult implies that the subject is in their 20s whereas a child would be much younger than that. Doing a back flip on a trampoline near a lake requires the subject to be on the trampoline, whereas jumping into the lake would require the participant to be in the water.
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