12. 於口腔毛狀白斑症(oral hairy leukoplakia)之組織切片中,氣球樣細胞(balloon cells)通常出現於上皮之何處? (A)角質層(keratin layer) (B)上棘狀層(upper spinous layer) (C)下棘狀層(lower spinous layer) (D)基底層(basal layer)