If the vectors a¯i+¯j+¯¯¯k,¯i+b¯j+¯¯¯k,¯i+¯j+c¯¯¯k (a,b,c are not equal to 1) are coplanar, then
1/1−a+1/1−b+1/1−c=.....
a. 0
b. 3
c. 2
d. 1