What does an Estimator require that Sampler usually does not?

Question: In Qiskit-style primitive workflows, which input is naturally associated with Estimator but not with a basic Sampler workflow? A) An observable whose expectation value is to be estimated B) A classical bit register for every measured qubit C) A fixed number of shots for every circuit D) A transpiled circuit with only single-qubit gates Correct: A Explanation: Estimator evaluates expectation values of observables for quantum circuits. Sampler focuses on sampling output bitstrings or probability distributions from circuit measurements. Both may use circuits and shots, but the explicit observable is central to Estimator. Topic: advanced quantum computing / Qiskit primitives / Estimator vs Sampler