Chemistry · Structure of atom

Consider the following statements regarding stability of subshells:\n(A) Half-filled and fully filled subshells possess extra stability.\n(B) Symmetrical distribution of electrons leads to lower shielding of one electron by another.\n(C) Exchange energy is maximum when the subshell is either half-filled or fully filled.\n(D) Chromium and copper exhibit anomalous electronic configurations due to extra stability of \( d^5 \) and \( d^{10} \) states. \nHow many of the above statements are correct?

1. Only two
2. Only three
3. Only one
4. All four