A:to blame; occurred
B:to blame; occurring
C:to be blamed; occurred
D:to be blamed; occurring
下列各项中属于青蛙的呼吸特点的是( )
关于长江、黄河的说法,正确的是( )
已知直线l1:ax-y+2a=0,l2:2a-1x+ay+a=0互相垂直,则a的值是( )