Bad Qubits — Learn Quantum by Doing

Multiple choice

In the EPR argument applied to a spin-singlet pair, EPR concluded that both S_z and S_x of Bob's particle have definite values. Why does this clash with quantum mechanics?

The singlet state is not normalized, so probabilities cannot be assigned to S_x and S_z
S_x and S_z are incompatible observables: their operators do not commute ([S_x, S_z] = -i hbar S_y is nonzero), so no quantum state assigns both a sharp value
Quantum mechanics forbids measuring spin along any axis other than z
S_x and S_z commute, so quantum mechanics already assigns both sharp values and there is no clash