20. 內弓狀纖維 (internal arcuate fibers) 是直接源自於下列何者，並跨過中線到對側的上行纖維？ (A) 弓狀核 (arcuate nucleus) (B) 三叉神經脊髓核 (spinal trigeminal nucleus) (C) 薄束核及楔狀核 (gracile and cuneate nuclei) (D) 外側網狀核 (lateral reticular nucleus)