= Solution
A <unit cell> is a repeat region whose lattice translations tile the plane. A <primitive unit cell> represents exactly one <lattice point> and has the minimum repeat area for a lattice basis.
\b[Cell A is nonprimitive; cell B is primitive.] A represents two points and has area $4a^2$. B has four shared corner points and no additional interior point, represents one point and has area $2a^2$. Fractional corner counting refers to the tiling, not to fractional physical atoms.
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