{
  "correct_letter": "B",
  "correct_formula": "ΣF_ext = 0",
  "annotation": "Environment: g=-9.81, F_drag=0.0",
  "choices": [
    "n_1·sinθ_1 = n_2·sinθ_2",
    "ΣF_ext = 0",
    "G = m·g",
    "τ_net = Σ(F_i·r_i·sinθ_i) · e^(-μ·t)"
  ],
  "correct_index": 1,
  "physics_type": "AtRest",
  "seed": "AtRest|benchmark/at_rest/2"
}