When assessing a client as a candidate for crutch walking, the nurse should take into account that for some elderly people, crutch walking is an impractical goal primarily because which of the following reason

A. Decreased reaction time.
B. Decreased visual acuity.
C. Decreased motor coordination.
D. Decreased level of comprehension.

脑及脊髓内的（）细胞破坏后不能再生，由（）细胞及其（）修补，形成（）。