双子叶植物根的初生结构中，内皮层细胞径向壁和横向壁四面加厚，这种呈木栓化的加厚称为( )。
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被子植物维管束中的导管发育成熟时，端壁部分被溶掉而形成穿孔，穿孔可分为( )和( )两种。
双子叶有胚乳种子由( )、( )和( )三部分构成。
We can conclude fromthis passage that ballet
A. is a dying art
B. will continue to change
C. is currently performed only in Russia
D. is often performed by dancers with little training
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