A:-∞,-1
B:1,+∞
C:-1,1∪1,+∞
D:-∞,+∞
设函数fx=x2-8x+c1x2-8x+c2x2-8x+c3x2-8x+c4,集合M=x|fx=0=x1,x2,⋯,x7⊆N*,设c1≥c2≥c3≥c4,则c1-c4=( )