For a quadratic form f(x, y, z) ∈ ℝ[x, y z], we say that (a, b, c) ∈ ℝ3 is a zero of f if f(a, b, c) = 0. Which of the following quadratic forms has at least one zero different from (0, 0, 0)?
1
x2 + 2y2 + 3z2
2
x2 + 2y2 + 3z2 - 2xy
3
x2 + 2y2 + 3z2 - 2xy - 2yz
4
x2 + 2y2 - 3z2