Let α ∈ R and the three vectors a = αi + j + 3k, b = 2i + j - αk and c = αi - 2j + 3k. Then the set
S = {α : a, b and c are coplanner }
(1) contains exactly two numbers only one of which is positive
(2) is singleton
(3) contains exactly two positive numbers
(4) is empty
