黄酮类化合物的UV光谱中产生吸收带Ⅱ的基本结构是
A.乙酰基B.苯甲酰基C.咖啡酰基D.桂皮酰基E.阿魏酰基
参考答案:B
如图,P、Q两点有两个频率相同的声源,它们激起的声波波长为2m,P、Q到O点的距离分别为1m和6m,那么在x轴上从+∞到-∞的位置上,会出现的声音减弱的区域有几个?
I'm sure that stricter roles to control cigarette smoking _____very soon. [ ]
A. made
B. will make
C. were made
D. will be made