A:collect
B:to collect
C:collecting
D:collected
The________expression on his face suggested that he was________at the extraordinary beauty of the picture.
异戊二烯是重要的有机化工原料,其结构简式为CH2=C(CH3)CH=CH2.
完成下列填空:
若x,y为实数,且|x+2|+y-2=0,则(xy)2013的值为( )