19.正常人的腦脊髓液主要由何處產生？ (A)腦室內脈絡叢(choroid plexuses) (B)腦蜘蛛膜下腔(subarachnoid space) (C)腦室的室管膜細胞(ependymal cells) (D)骨髓(bone marrow)