code: def get_under_bridge(self):
          
          C0 = self.B[-1:] * (3, 3, 1)
          ads_pos = C0.positions[4]
  
          C = self.get_subsurface_layer() * (3, 3, 1)
          dis = self.B.cell[0][0] * 2
  
          ret = None
  
          for ele in C:
              new_dis = np.linalg.norm(ads_pos - ele.position)
              if new_dis < dis:
                  dis = new_dis
                  ret = ele.symbol
  
          return ret
docs: Return element closest to the adsorbate in the subsurface layer