Stock base material selection

Hellow SAP Gurus,

Here i have 2 roh material for 1 finish material. E.g A is my finish material for that i required 1 Roh R1/R2 means either R1 or R2. here R2 is Alternative material for R1.Now if i have R1 of 10Qty & R2 is 0 stock So after MRP run for material A 10Qty is it poosible system select R1 not R2 because R1 is in stock?

regards