单项选择题 3．下述食品污染物中，（   ）属于食品的杂物污染 A. 131碘 B. 铅 C. 铁屑 D. 有机磷农药

单项选择题 ‍砌体材料中块体的表示符号为？‏ A、M B、MU C、Mb D、Mc E、Me

单项选择题 Which is true about Embryonic Stem Cells.  A、They no longer do mitosis, their cell type is set and can not change B、 They are limited to the type of cell they can become C、They can become any cell type in the body D、They are found in adults E、All of the above.