请教 OG208 Teratomas are unusual forms of cancer because they are composed of tissues such as tooth and bone not normally found in the organ in which the tumor appears.
(A) because they are composed of tissues such as tooth and bone
(B) because they are composed of tissues like tooth and bone that are
(C) because they are composed of tissues, like tooth and bone, tissues
(D) in that their composition, tissues such as tooth and bone, is
(E) in that they are composed of tissues such as tooth and bone, tissues
答案E. ,tissues not normally founding te organ in which the tumor appears是独立主格结构吗?否则tissues前面的逗号是什么意思?