蒽醌类成分按母核的结构可分为（）和（）两大类。
参考答案：单蒽核；双蒽核
-- Can you persuade your brother to go with us?
--- No, he will never do ______ he is asked to.
A. whatever B whenever C. however D. whichever
心肌不会产生强直收缩，其原因是( )
A．心肌是功能上的合胞体B．心肌肌质网不发达，Ca2+贮存少C．心肌的有效不应期特别长D．心肌有自律性，能自动节律收缩E．心肌呈“全或无”收缩
