Q: In this task, you are given two facts, and a multiple-choice question. Based on the given facts, answer the question with index of the correct option (e.g, "A").
Fact1: Ancestors of hagfish are thought to have been the earliest vertebrates., Fact2: Vertebrates are animals with backbones., Question: Ancestors of hagfish are thought to have been the earliest (A) filter feeders (B) unicellular organisms (C) carrying offspring to birth (D) animals with backbone (E) last stage before adulthood (F) prokaryotic cells (G) To survive in their environment (H) inherited antigens + antibody
A:
D