Which of the following statements is correct for alpha particle scattering experiment ? (A) For angle of scattering $\theta \approx 0$, the impact parameter is small. (B) For angle of scattering $\theta \approx \pi$, the impact parameter is large. (C) The number of alpha particles undergoing head-on collision is small. (D) The experiment provides an estimate of the upper limit to the size of target atom.