Why is probabilistic error cancellation high overhead?

Question: Probabilistic error cancellation represents ideal operations using signed quasiprobability combinations of noisy implementable operations. What causes its main sampling cost? A) It requires exact statevector simulation of every circuit B) It increases qubit count by one ancilla per noisy gate C) It forces all gates to be Clifford gates D) Negative or large quasiprobability weights amplify estimator variance Correct: D Explanation: PEC can be unbiased under an accurate noise model, but signed quasiprobability coefficients must be sampled and recombined. When the sum of absolute coefficients is large, the variance and required number of circuit samples can grow sharply. Topic: advanced quantum computing / PEC / error mitigation