Alternative item problem

Dear Sir

i have one problem and i search about it and not found solution for it that is :

I have material X FERT and contain in it's bom two material A and B , and material C will be alternative for material B

i create alternative item group Z as below

My problem is  if requirement for material X is 100 and stock of material B equal 99 so system not consider this stock and jump to second priority

My question if there any method that let the  system will reserve stock from mat. B until to be zero and after that jump to second alternative material ?

regards

Nassar