In many organisms other than vertebrates, and in all vascular plants, another reaction sequence, the _____________, serves as a mechanism for converting acetate to carbohydrate.

下面哪些句型，通常被用来引出研究问题？ A: While XYZ is well documented, it remains unclear whether/why... B: Despite a large body of evidence for XYZ, its detailed mechanisms remain to be elucidated. C: However, it remains to be investigated whether... D: ABC is the main mechanism used by all vertebrates to achieve XYZ.