用水平风吹如图四个模型,空气对模型下表面的压强大于上表面的压强的是( )
A:
B:
C:
D:
- Has David finished his project yet?
- I guess not. He ___________ on it at lunch time. Do you want me to confirm it?
“猪——沼气——作物”是常见的生态农业模式,由种植(作物)、养殖(猪)、农户(人)和沼气生产(分解者)四部分组成。有关该生态农业模式的叙述正确的是( )