煤的结构很复杂,是以()为主具有烷基侧链和含氧、含硫、含氮基团的高分子化合物。
参考答案:芳香核结构
Everything he _____ to his country. [ ]
A. had was contrbuted
B. had been contributed
C. had to be contributed
D. had contributed
如图所示,把一小球从碗壁a处放开,小球经a点运动到b点,在这一过程中,小球的能量转化情况是:
A.从a到O,动能转化为重力势能,从O到b,动能转化为重力势能
B.从a到O,动能转化为重力势能,从O到b,重力势能转化为动能
C.从a到O,重力势能转化为动能,从O到b,动能转化为重力势能
D.从a到O,重力势能转化为动能,从O到b,重力势能转化为动能