Which of these is NOT a vector space?
The set of polynomials of degree 2
The set of 2×22 \times 22×2 matrices
The set of all functions f:R→Rf: \mathbb{R} \to \mathbb{R}f:R→R
The set of all points (x,y)(x, y)(x,y) such that x+y=1x+y=1x+y=1