A:of protect
B:protecting
C:to protect
D:to protecting
下列哪种细胞中染色体成单存在( )
—我们禁止在马路上跑。
—What____you not do to____the road ____?
—We ____ ____on the road.
Why not try on that pair of ____?
做匀速圆周运动的物体,改变的物理量是( )