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RIGOR MORTIS (stiffness after death) occurs because:

AExcess calcium builds up in muscle cells
BLactic acid hardens the muscle proteins
CBones dry out and lose flexibility
DLack of ATP keeps myosin attached to actin
Answer & Solution
Correct answer: D. Lack of ATP keeps myosin attached to actin
After death, ATP production stops. Without ATP, myosin heads cannot release from actin → cross-bridges stay locked → muscles stiffen (RIGOR MORTIS). Resolves after 24-48 hours as proteins decompose.
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