= Solution
\b[Either experiment is possible.] In the sequential experiment choose $U=X$. The first <computational basis> measurement on $I/2$ returns a uniform <bit> $j$ and prepares $|j\rangle$. The <Pauli X gate> changes it to $|1-j\rangle$, so the second result is always opposite. The two possible records have <probabilities> $\Pr(0,1)=\Pr(1,0)=1/2$.
In the two-<qubit> experiment choose the fixed <pure state>
$$
|\psi\rangle=|\Psi^+\rangle=\frac{|01\rangle+|10\rangle}{\sqrt2}.
$$
The <Born rule> gives the same two possible records, again with <probability> $1/2$ each. The observed <computational basis> anticorrelation alone does not identify whether the correlation was generated sequentially or shared between two <qubits>.
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