—Your room is in a mess, David.
—I'm sorry. I'll __________ right now.
A:pick up
B:clean it up
C:clean up it
D:turn it up
一个载重气球在空气中匀速上升时,受到的浮力为2000N,若在所载重物中再加200N的物体,这时气球就能匀速下降,假设气球上升和下降时所受的浮力和阻力大小不变,则气球的重力为 ____ N,受到的阻力为 ____ N.
阅读图的某地等高线地形图,回答下列问题.