C
定义域：
x+1＞0
x-1＞0
x＞1
y=log2(x+1)+log2(x-1)
=log2(x^2-1)
设x1,x2∈(1,+∞),且x1＜x2
y1-y2=log2[(x1^2-1)/(x2^2-1)]
(x1^2-1)/(x2^2-1)＜1
log2[(x1^2-1)/(x2^2-1)]＜0
y1-y2＜0
单调递增