Let a=2i+j+k,b=i+2j−k, and a unit vector c be caplanar. If c is perpendicular to a, then c is?
a. 1/√2 (-j+k)
b. 1/√3 (-i-j-k)
c. 1/√5 (i-2j)
d. 1/√3 (i-j-k)