题目

You were ________ to close the windows. Why were you so careless? A. allowed     B. believe       C. supposed     D. caused 答案：C若精子中DNA含量为a，则初级精母细胞和次级精母细胞的DNA含量分别是 （   ） A．2a和a           B．4a和2a           C．a和2a           D．2a和4a