A:{﹣1,0,1,2}
B:{﹣1,0,1}
C:{﹣2,﹣1,0,1}
D:{﹣2,﹣1,0,1,2}
显微镜观察草履虫时,经常在临时装片的培养液中放一些棉丝,其作用是( )
— We did it by .