The key here is open your mouth wider for the uh in bom no B at the end that it's a silent letter so open wide for the ah bom bomb bomb and your mouth will remain relatively closed.
這裡的關鍵是為bom中的uh張大嘴,最後沒有B,這是一個無聲的字母,所以為ah bom bomb bomb張大嘴,你的嘴會保持相對封閉。