GMAT Sentence Correction (SC) | While digging in the Egyptian desert, huge fossil bones have

Himikemcgarry

Sorry to bother. Could you please help out? I need to understand a concept
Here

D: paleontologists have found huge fossil bones from what appears to be

Are we using singularappears because it refers to singular the second most massive dinosaur ?

or
what appears to be is idiomatically always singular becauseappears refers towhat , which is unknown and always takes singular form?

Thanks in advance

Best
RzS
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