can you please explain how to answer this:

Question

Which of the following is not a plane
a) r=(1,3,4)+s(2,-1,2)+t(1,1,1), s,t∈R
b) r=(2,4,2)+s(1,-2,3)+t(3,2,2), s,t∈R
c) r=(3,2,3)+s(4,-4,2)+t(-2,2,-1), s,t∈R
d) r=(-2,1,4)+s(2,2,-1)+t(2,2,1), s,t∈R

Answer 1

in (c) the two s- and t-vectors are collinear.