刨料：门窗框料选一个（）为正面，门窗扇料选一个（）为正面，正面应划出符号。
参考答案：窄面、宽面
I have __________ the problem for two hours, but I haven't __________.[ ]
A. worked on, worked out it
B. worked at, worked it out
C. worked on, worked it out
下列黄酮类化合物C环质子的1H-NMR信号
质子信号出现在最低场(δ7.82，s)的是
A．山柰素 B．芹菜素 C．大豆素D．二氢山柰素 E．二氢芹菜素
