A:乙炔
B:丙炔
C:丁炔
D:丁二烯
使用显微镜对光时,下列哪些结构应在同一条直线上( )
He turned from side to side, eyeing himself in the mirror with a serious, studied ______ that soon changed into a smile.
已知a、b、c均是不等于0的有理数,则aa+bb+cc+abab+bcbc+acac+abcabc的值为____