Assume that rotation about the inter-ring bond of a biphenyl is blocked when all four ortho positions carry substituents and free otherwise. Which of the following biphenyls are chiral?

Axial chirality requires blocked rotation (four ortho substituents here) and two different ortho groups on each ring. A: each ring has $\mathrm{I}$ and $\mathrm{CH}{}_3$ ortho: chiral. B: each ring has two identical $\mathrm{CH}{}_3$ ortho groups: a mirror plane persists, achiral. C: only three ortho substituents, so rotation is free by the stated rule (and one ring would have identical ortho groups): achiral. D: each ring has $\mathrm{NO}{}_2$ and $\mathrm{COOH}$ ortho and all four ortho positions are occupied: chiral. Correct: A and D.