Why do T gates dominate fault-tolerant resource estimates?

Question: Why are T-count and T-depth often emphasized in fault-tolerant quantum resource estimation? A) T gates are usually non-Clifford resources and often require costly magic-state distillation or injection B) T gates are the only gates that can be measured directly C) T gates are classically simulable while Clifford gates are not D) T gates replace syndrome extraction in surface-code architectures Correct: A Explanation: Clifford operations are comparatively easier to handle fault-tolerantly in many architectures, while non-Clifford gates such as T often require expensive magic-state resources. T-count measures total use, while T-depth measures sequential layers of T operations. Topic: advanced quantum computing / Clifford+T / fault-tolerant resources