答案：D

句意：这个花瓶要是底部没有那几个小洞就完美了。“but for a few small holes in its base”表示含蓄的条件，相当于“if there were no small holes in its base”。根据句意可知，此处应为与现在事实相反的虚拟语气，故主句用would be，所以选D项。