B
X
1
X
2
⋯
X
m
X
1
c
11
c
21
⋯
c
1
m
X
2
c
21
c
22
⋯
c
2
m
⋮
⋮
⋮
⋮
X
m
c
m
1
c
m
2
⋯
c
mm