C-12-Q081mediumsingle_mcqWith the configuration in Q80, PQ⃗=\vec{PQ}=PQ=a12(AD⃗−BC⃗)\frac{1}{2}(\vec{AD}-\vec{BC})21(AD−BC)b12(BC⃗+AD⃗)\frac{1}{2}(\vec{BC}+\vec{AD})21(BC+AD)c12(AB⃗−BC⃗)\frac{1}{2}(\vec{AB}-\vec{BC})21(AB−BC)d12(BC⃗−AD⃗)\frac{1}{2}(\vec{BC}-\vec{AD})21(BC−AD)