Under PPO, which of the following in-network specialists would a referral be required?

PPO: You do not need a referral to see a specialist. However, some specialists will only see patients who are referred to them by a primary care doctor. And, some PPOs require that you get a prior approval for certain expensive services, such as MRIs.

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I believe the answer is: C.Producers convert light energy into chemical energy

As organisms move up higher on the trophic level, they would become much more dependant toward consuming other organism to obtain their energy.
Producers who placed the lowest on the trophic level could produce energy without consuming other organism due to photosynthetic ability.