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").
Q: Fact1: Neurons are the structural and functional units of the nervous system., Fact2: Glial cells perform critical functions in the nervous system., Question: Neurons are the structural and functional units of where what is critical? (A) organs (B) Glial cells (C) nerves (D) Structural units (E) voltage (F) Storage units (G) Critical care (H) A computer
A:
B