正确答案: D

一层或二层

题目：珠被常为( )。

解析：D

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举一反三的答案和解析：

[单选题]DWhen I first got an email account ten years ago, I received communications only from family,friends,and colleagues.Now it seems that every time I check my e-mail ,I have an endless seriers of advertisement and otner correspondence that do not interest the at all.If we want e-mail to continue to be useful,we need special laws that make spamming(发送垃圾邮件) a crime.

To persuade

[多选题]被子植物的木质部是由( )构成的。

导管

管胞

木纤维

木薄壁细胞

解析：ACDE