ACCorDing to rEsEArChErs, riChArD nixon's DEAth wAs _______.
A．CAusED By his hEArt proBlEms.
B．inDirECtly linkED to his wiFE's DEAth.
C．thE inEvitABlE rEsult oF olD AgE.
D．An unExplAinABlE ACCiDEnt.