下列何者為小腸之內襯上皮？(1)無纖毛之單層柱狀上皮(nonciliated simple columnar epithelium)(2)單層扁平狀上皮(simple squamous epithelium)(3)偽多層柱狀上皮(pseudostratified columnar epithelium)(4)移形上皮(transitional epithelium)(A)(1)(B)(2)(C)(3)(D)(4)